Tier 1, da best
Rogue, BM, Windwalker, Arms, Shadow
Arms may be high tier 2, but I won't believe it until 5.2 is live for a while and warriors have time to adjust to not playing like retards. Shadow is still on the chopping block, but I expect it to land in tier 1 or high tier 2.
Tier 2, good
Frost Mage, Ele, Warlock
Ele is gonna be way better, but I'm not ready to jump on the "best caster" bandwagon just yet. Warlocks are difficult to place because, even after individual nerfs, their synergy with other classes is always stellar.
Tier 3, so-so
DK, Feral, Ret, Enhance, Moonkin
I think the gap between the haves and the have-nots will be a little lower in 5.2; these specs won't be bad, just not quite as good. Unholy is looking to make a comeback. Feral is getting nerfed harder than people think IMO. Ret is getting some nice healing buffs, but it may still be too easy to zerg in 3's.