I'm going to start with a look at a bunch of cards I've found interesting or mis-evaluated by lots of people, or which have some math-y component to tell you how good they are. Let's jump in!
Walker of the Wastes
This is a card that I seem to see getting vastly overrated more or less everywhere. 5 mana for a 4/4 trample is, first of all, not a very good body, I think. And I suppose this is a big chunk of the disagreement, so let's go over it for a bit. 2/3 is a very common body size here. I think it's going to be pretty often that the board will stall a bit with some 2/3s and 2/4s and maybe 2/2s and 2/1s laying around. In those spots, a 4/4 just doesn't do all that much for you - you can attack and trade it for a 2/3, you can leave it back and hold off their team, you can use it in conjunction with a trick to potentially get a blowout, but of course a 3/3 is just as good in all of those scenarios, and to some extent this can be played around. Of course, I would rather have the 4/4 than the smaller creature in these situations, but the far bigger point to my mind is the mana cost. You can only run so many 5-drops, space is at a premium, and I'm going to want the lower curve card just about every time,. Indeed, I think it's usually worse to have too many low drops than it is to have too many high drops. By the way, it's for these reasons that I really don't think Cinder Hellion is very good at all.
With Walker in particular, though, there's some chance of getting paid off. A 5/5 Trample for 5 would indeed be good, as you're reasonably likely to get a 2-for-1 off of it, or at least hold off most anything (5/5 is, as we saw in the last part, bigger than the vast majority of cards in the format - something that wasn't true of 4/4s). And if you can get it any bigger, then it's obviously very, very good. And I think this is the other big problem I am having with other people's evaluation of this card. You're just not going to have enough Wastes to make this realistic. There are 2 common slots devoted to Wastes, which equates to about 4.5 per draft. There's also lots of other colorless sources, and besides the possibility of a couple basic-searchers (Loam Larva and cards from BFZ mostly), you're going to prefer just about every other colorless-producing land basically every time, as they're more or less strictly better than Wastes. Still, the number of repeatable colorless sources is limited, so I wouldn't expect to get more than 2 or 3 Wastes with any kind of frequency. 2 is the point at which I think I would start being ok running the Walker, but I'd need 3 or 4 before I would think it was good. I don't find this very likely, but there you go.
Warping Wail (AKA Eldrazi Charm)
This card is getting a lot of hype for constructed, perhaps rightly so. I'm not always the greatest prognosticator of metagames (even if I do know that prognosticator is a great word). But I do know that it won't be a very good limited card. It does kill a bit under a third of the creatures in the format, but lots of those are unplayable, and several cost 1-2 mana while not being super impactful - you know, ye olde Goblin Piker. In terms of sorceries to counter... well, there just aren't so many that your opponent is likely enough to have one that you can hold this up. Making a Scion isn't going to be worth the card either, though if you do end up running it for whatever reason, having that option can be nice. This is hard to cast, and it just doesn't do much. Keep it in the board, maybe bring it in if they have enough little utility creatures and/or bomby sorceries.
Ondu War Cleric
I think a lot of people fail to realize how good two life per turn is. No, this isn't a great card, but it's very solid, I'll play it every time I'm white.
This removal spell is highly conditional. You need their creature to be attacking or blocking, and you need it to be small enough. These of course limited the usefulness of such a card; you often don't care about creatures that small, and when you do, they're often utility creatures, which are going to be harder to get in combat anyway. Still, as we saw last time, this kills slightly over half the creatures in the format, which is nothing to sneeze at. At 1 mana, I imagine it's going to be fine, but I would never expect it to be great, as you're just not going to get an amazing deal. Definitely better in some matchups (e.g. against walls), so think about it whenever you're sideboarding. Also keep in mind that this can blow out or be blown out by combat tricks.
Grasp of Darkness
A note on this as well as the next card down. You probably aren't going to get to cast this on turn 2 very often, both because getting to BB is hard, and because your opponent probably won't have played anything you want to waste this premium of removal on so early in the game. However, just because you won't want to play it until turn 5 or 6 very often doesn't mean that you're losing out on the value of the 2 CMC: these kinds of things are prime candidates for double-spelling. This plus a three-drop (usually of your other color, I would guess) on turn 5 is going to be game quite often. And it does kill most of the cards in the format (as well as being able to be used as a combat trick against virtually all the rest).
Inverter of Truth
Really not a 4 mana 6/6 flier in the sense that makes you think, since playing it turn 4 is going to be a disaster basically every time. There is a combo here with Corpse Churn, but I don't particularly imagine that's where you want to be. But it's worth noting that if you do play this late, 6/6 flier still outclasses most everything, especially in the air, and you are going to be drawing gas the rest of the game. Not that this is spectacular because of that, but the ability isn't 100% downside (only like 90%).
Visions of Brutality
Obviously, this card tops out at Pacifism - with a few exceptions, they can just choose to not attack and be no worse off than if the card was Pacified (watch out for forced attacks and fight cards primarily). On the other hand, Pacifism is very good, and how much worse is this, really? The biggest thing this card has going against it is that it's a triggered ability - that means that if the creature getting through would mean 'double lethal' the player attacking with it will win, and the one whose Visions it was won't be happy. This can be dealt with via chump blocking, but then, you need a blocker available.
We can look at more normal scenarios, though. To make this worse than Pacifism, that means you must be attacking with the creature. If you're going to do that, you're making the game a race; this is something Aggro decks generally want to do, and controlling decks don't, which suggest that this is a card you'd rather play in an aggro deck than a control deck. However, I don't think it's too bad in a control deck, either - they had to be losing the race fairly badly to still be losing once their best creature is playing for both sides. This problem is especially exacerbated if the control deck has some chump-blockers. A random bear may not often be what you're really looking for in these kinds of decks, but you tend to need some defensive speed, and if you get to turn it into a drain 4 while racing, that's a big game.
This is one of my favorite cards in the set, from the looks of it, and definitely my pick for sleeper. I'm very hyped about this card, and... nobody else seems to be. First of all, a Piker is just fine. But then you get to activate this, possibly every turn, which seems like a big game. Its ability stops 40% of the creatures in the format from blocking, and more important, these are the creatures that you are usually most annoyed with, as an aggro deck. What's left is likely to trade off, leaving the opponent with even less to deal with the next attack. The ability also works with Menace well. Of course this isn't going to be a bomb, but it's something I'll really be looking for in my aggressive decks.
Maw of Kozilek
This card seems simply quite good to me. 2/5 for 4 mana is solid already, and the activated ability does a lot of work. If it cost generic mana rather than Colorless proper, I think that would make this one of the better commons in the set; as is, it's probably 'only' a very solid, above-average card.
I think this card is getting overrated, probably because it's going to be a lot easier to set up in Constructed. If you aren't getting a spell back with this, it's quite mediocre. (Indeed, it was a common in BFZ that was only marginally playable despite having a relevant subtype). Of course, I imagine I would run this with even a few hits in my deck, as it's not that far off even without hitting. But for it to really be good, you want it to be 'on' most of the time, and relevantly so (i.e. combat tricks that the opponent can clearly see coming lose a lot of value). I would guess you would want 4, 5, or even 6 spells that fit this criterion for that to happen reliably. 4 is probably doable, especially if you're looking for them after getting this fairly early, but 6 isn't terribly realistic, given that you're only probably going to have room for 8 non-creature spells max. Definitely I expect this to be a good draft card, but I could also definitely see passing it for good uncommons or even a few of the top commons.
These kinds of cards are very typical for red to get as 'build-around-me' thematic deals for red. It seems we get them about one per block - BFZ had one that triggered off colorless spells, Origins wanted artifacts in play, Journey Into Nyx had you triggering off scrying, Innistrad off flashback... there are others. The problem is, unlike 'the red common burn spell' which is almost always at least good and often excellent, these cards vary incredibly wildly in their power level.
This one shocks creature or player. That's good. You can only do it once per turn, which is a little knock, because it means you can't get it to take out something bigger (though potentially you could combine this shock with one of the two spells triggering it to get such an effect). It's also rewarding you for doing something you're going to want to do anyway, which again, is good. I assume you'll be happy with the card if it triggers twice. Any trigger is going to kill less than half the creatures in the format, but it's a big enough chunk that I imagine you'll still have some target usually. Killing two things, even little things, is going to be worth your 4 mana. More will make it very good. Only one is going to be pretty mediocre.
The best case is that you play this and follow up with another spell in the same turn, thus not wasting any potential triggers. Unfortunately, even red doesn't have all that many 1 mana plays, so it will probably be quite late before you could do that. So probably fairly often, you'll run this out there and hope to trigger off multiple things later. The big problem I see with this card is that you need the game to go fairly long for this to work out and get enough value, but you also need cheap spells to trigger it, and you need to not have dumped too much of your hand by the time this gets down, and these things are kind of at odds. So it will be pretty hard in order for this to get working, and then you need a pretty good flow of cards (or enough time to wait to naturally draw enough). I expect most of the time, this card will actually end up being quite bad, but if you can get it online, of course it can be great. I'm more apt to let my opponents be going for that sweetness, though.
Boiling Earth was maindeckable (at least arguably). There are going to be way, way fewer scion-producers now, though, and fewer X/1s in general. *Eldrazi Skyspawner much rarer now cough cough cough*. So you definitely don't want to play this to try to kill things - you need the falter effect to be good, and that means I don't think you should play it very often at all - when you do, it should be because you're really aggressive.
Tears of Valakut
Is this main-deckable? There are 34.6 fliers per draft in the set, which works out to a little more than 4 per player. Some of these are pretty miserable, and you wouldn't really care about killing them anyway. A very few are too big to be taken out by this. So, on average, you're probably looking at 2-3 targets per deck worth hitting. 3 Targets would be enough to play, 2 would be borderline, but the bigger problem is that I think there are probably too many decks with 0-1 targets, so I'd really prefer to start this in the board, and look to bring it in fairly often. Things are a bit different in sealed, where I'd be more likely to play it main, as evasion and removal are both better there.
I expect this to work reasonably like fight cards have in the recent past. It's a spell that's a bit risky, vulnerable to blow out, can get one of their creatures if it's the only thing left to block, or as an ambush blocker, and is limited on size to what it can take out. Having said that, 4/4 is a reasonable size. But this is still probably worse than most of those, because the line of play you often want to take is to kill their thing and attack with yours (hopefully with extra damage coming in, as was the case in Tarkir), which you can't do here. This also has a harder time dealing with evasive threats, which fight spells are usually pretty good at. And of course, it usually effectively costs 3 mana rather than 2. To make up for that, you occasionally get to Boros Charm their face. No, that's not enough to make up for the other deficiencies, but I expect this to be fully playable nonetheless, and to almost always make your green decks.
I expect this card to be very very good, one of if not the very best Uncommon in the set. 3/4 for 4 is just fine to start with, getting a 1/1 on death is already good even, and very often you'll be getting a 2/2 or a 3/3, which is getting into the fantastic range. Picking up some ginormous creature on the back end is probably not something you can count on, but you can do some maneuvering, and it won't be super rare anyway. The biggest downside is that so much of the removal here exiles, but even so, this thing will never be bad, and so often it will be SO good.
I just want to say, I can't wait for this to join the Cube.
Very very good card. Slam it turn 3, attack turn 4, you're always going to have threat of activation up, it will do a heck of a lot of work for you. Also, you're not limited to once per turn, so later on this is a really good mana sink. I'm not sure that I wouldn't rather have this in play on turn 6 than a 5 mana 4/4 trample, and it costs a whole 2 less mana.
Another card that I think is being significantly overrated. Again, it's probably a lot better in constructed. In limited 1/3 fliers for 2 are fine, but kind of take-it-or-leave-it. If you're straight 2 color, you'll probably have UR on turn 2 reasonably often, but on the other hand, you're pretty likely to be splashing or wanting colorless now, so you certainly might not. The haste isn't worth so much on a 1 power creature (though I suppose it's not nothing). And then the question is, how good is prowess. It's fine, but while I don't think you'll cut this from your UR deck almost ever, it's not something I'd want to go out of my way for either.
I don't understand why people think this card is going to be good. We all realize that Bone Saw is bad, right? A big amount of the time, this is (albeit very marginally) worse than Bone Saw, right? For this to be good, you need to be able to equip it and attack, for your attacking creature to not get eaten, and for the 1/1 created to not get eaten. When this only gives +1/+0, that seems like a fairly tall order. Sure, if it gets going early, it can steamroll a bit, but I am just not seeing that best-case-scenario pop up often enough... I don't think I would run this card in most decks. Of course, it does have synergy with Rally, but in terms of relevant Rally triggers, you're looking about 2 being opened per draft. (Of course this is better than Bone Saw overall, and there will be more decks that want this than want to actually run that card, but I would not be excited by this thing in the slightest.
This, on the other hand, is the real deal. I am not going to run down every tribe, but I really don't need to. What you need to know is there are by my count 64 Eldrazi plus scions per draft, and 54 Allies plus some tokens per draft. I don't expect decks to be as all in one or other as they were in BFZ, but I still generally expect one vs the other to be fairly predominant, meaning that I would guess most decks have at least 10 cards in the same tribe. That, in turn, should make it pretty easy for this thing to give at least +2/+2, at which point it's a Vulshok Morningstar. That might not seem like a crazy thing to you, but I assure you, that's very, very good. If the format ends up leading to board stalls, bigger bonuses will ensue, at which point this card will just win you the game. It's very, very good, possibly the card I would most want to open P1P1 (definitely not the strongest card in the set, but it does leave you open to any deck... probably there are a few others where the power is just too much higher, but this is up there).
General consensus is that Red is the worst color in the set, followed by Green. People think Black looks the best, probably followed by blue.
I definitely agree that Black is the best color, and by a large margin. But I think I would actually peg Green as second-best, followed by Blue, Red, and White. Having said this, I think the difference between Green and White (2nd-best and worst) is smaller than the gap between Black and Green (best and 2nd). I also don't think that Black is good enough to force in any respect.
It's worth noting that because Green is so bad in pack three, reading it as open late in pack one has much worse pay-off, and isn't really better than cutting it in pack one - you want it to be open in one and two, and care about it less in three. Having said that, there are green cards in BFZ that were totally fine, it's just that the color as a whole didn't come together. It's also worth noting that generically decent to big bodies from BFZ are probably going to be better now, since there's so much less synergy. Broodhunter Wurm is out of the dog house, and Plated Crusher probably moves up to actually being pretty good, since there's just less large things now.
Blue is a tempo color now. The best commons are the tapper, the stays-tapped surge spell, the bounce spell.... It also gets a bunch of fliers. Typical blue I suppose, but it doesn't really have straight defense of big things to hold down the ground at all - you need to be tricky.
The format will have significantly less synergy than BFZ. This is not a bad thing, from my perspective. It will be more along the lines of a KTK - there are going to be a lot of mana sinks, no particularly strong themes, but some minor synergies and build-arounds. There won't be nearly as much time to durdle around and do nothing, but the format won't be all-out aggro-blitz like Origins was, either.
You really want to be curving out and impacting the board. There are some aggressive Eldrazi decks out there, the Support mechanic which I think is getting way under-talked-about, and to some extent cohort and surge are both mechanics to push your cost down. This doesn't mean 7-drops are unplayable, but you do need to have some way to survive that long without getting crushed in return, and I don't think you can run 3-4 7s anymore as was not too uncommon in my BFZ experience