Let me start off by saying this is coming from the point of veiw of a Death Knight. We have laughable defensives and are easily bursted down.
Warriors- Pretty much in the same boat as us DKs, great burst, shitty defense. I would say warrior's defense is slightly better than death knights because of the ability to switch to a shield and defensive stance whenever you get in a tough spot, then you let second wind keep you up until you are ready to burst.
Elemental/Enhance shaman- Borderline FOTM, INSANE burst and decent off healing is pretty hard to counter. Hex is a really good CC, as it has a much higher damage threshold than poly. Other than that I don't know much, but what I do know, is if you see one of these boogers pop their CDs, prepare your anus! (or CC them)
Resto shaman- Desperately need a buff of some kind, the only way to succeed as a healer in pvp , is to have all your heals be instant-cast. Having all those castbars is like candy to a DK, you bet your sweet ass you will never get a cast off!
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PS3 will only let me type so much, sorry!
Hunters- DKs counter hunters really well, they get constantly slowed by chillblains which leaves them with nowhere to run. I see people complaining about hunters a ton, but im just not seeing why...
Ret Paladin- Borderline FOTM, burst unlike any class in the game, crazy off heals (I have seen back to back 100k flash of light from a ret in contenders) and on top of all that, they can kite like no other class has kited before. (honorable mention- immunity bubble.)
Mages- Not that overpowered really, as long as you don't sit in a deep freeze and let them burst you down. It's pretty easy to guess when they are about to nova so I just ams that. Survivability is alright, I would like to see a slightly longer hypothermia debuff, but that may just be me...