Destro sucks at the moment, at least on the PTR.
Demo got a decent overhaul, but still not enough to call it exciting. Logical and much needed would be more appropriate, but still not exciting. No Demo's ultimate spell, "Demon Bolt" is still nowhere to be seen.
Affliction is like the same old cougar with a new boobjob. Not really exciting, same old rotation and shit. Only difference being is you get to use Curse of Doom in PVP, but for now it doesn't spawn "Demonic Guardian" as it says in the tooltip. Swap Soul is another nice spell to help you spread your DOT's quickly and cost-effectively.
DK's get nice and balanced specs.
Blood is awesome if you are looking for high survivability in a Battleground. Like when you are the designated Flag Carrier for your premade. You don't get to kill much stuff with it like on live atm but it is still awesome.
Unholy is excellent for those who love pets and burst DPS. Gargoyle + Frenzy + Ghoul Transform + IBF + AMS and AMZ can make a pretty deadly combo. You take out most casters with great ease and you have some decent cooldowns against melee classes too. Properly glyphed and gemmed an Unholy DK can do a pretty decent job even against imba mages who usually nuke other people with Arcane Missles critting for 40k.
Frost adds some excellent CC utility and has some potential to be pretty powerful but I haven't tested it yet.
DK's also seem to benefit a lot from Reforging their primary stat into Mastery Points, unlike other classes where things depend from spec to spec.
Priests appear to be pretty fun, but their really exciting spells start at 83 and above so nothing much to say there.
I realize PTR is not Cata but at least it's a nice preview of what's to come. Hope some of you might find my input interesting.