Oh I know. Frost DKs hit like a truck, and they put on pressure for days. They're the definitive anti-caster melee class/spec this season. Between Necrotic Strike, Mind Freeze, Deathgrip, Lichborne, Chains of Ice, Anti-Magic Shell, and Strangulate...yeaaaah. Gimme some sweet gear to go with all of that, and I'll be your worst nightmare. Lol.
Some healing classes can lame me out, though. Disc. priests give me a lot of trouble, followed by mother****ing Holy pallies. The problems I face against the former is by virtue of their spec's passives (have fun bursting down a Disc. priest stacked to the ceiling with Mastery), and the latter is more in line with just the class itself (I.e. if you're not on point in pressuring and controlling the pally, he'll outlast you in a 2v2 or higher). Resto shammies are tricky, too. Dat Deep Healing Mastery.
Other classes/specs? It's a case-by-case basis. I only really worry whenever there's a Feral drood or a Sub rogue around. Cross your fingers, survive the initial burst, and disrupt their stuns; that's all you can hope to do. It's worse with rogues because they can easily reset the match; however, droods can survive thanks to Bear form, and that's almost as bad.
Smooth Criminal