After playing a bit,
Angelic Purge seems very good. For starters, it's the only common catch-all removal at least comparable to
Oblivion Strike and
Isolation Zone. RW aggro usually has lands to throw away, BW has Zombie tokens galore, and UW has Clues. And you don't want to use your premium removal too soon anyway, so you should be able to afford to play at least one, maybe two of these with no appreciable drawback.
I was very impressed by
Cathar's Companion. Any kind of combat trick makes it not die in combat, and if the trick also makes your other attacker/blocker live through combat and kill the opposing creature, you get a nice two-for-one. Sure, you need to be the aggressor, and keep them tapped out on the defensive to get the most out of it, but it is something to consider.
In blue, I was pleasantly surprised by
Stitched Mangler. Creature not untapping can be huge in helping to stabilize the board. I only played against blue, and the rest seems quite weak, with the exception of
Sleep Paralysis and maybe
Nibilis of Dusk, if you plan to be the aggressor.
Not played black either, but it does look solid.
Ghoulcaller's Apprentice is another very solid common, in addition to Weight and Crow, probably also Throttle. Very solid all round.
In my draft, I tried for RW aggro, and
Howlpack Wolf was very good. As was
Voldaren Duelist. That is one of the best ways to keep up the pressure, three hasty power plus falter on their best blocker. Burn, of course, is great as ever.
No opinion on green so far.