Usually its kiting classes that know how DK's work are the tought ones.
Of course, this pretty much applies to any class that knows DK's, and any DK that doesn't know his class can be beat. Leveling my prot warrior in BC-land on a PvP server, and let me tell you i have plenty of DK's death gripping me as im fighting a mob. It sorta funny when that do that...id prefer to be in melee range anyways, and their first disease hit gives me just enough rage for a shockwave. Fun stuff.
then a Dk comes along who knows his class...yuk :P i go down pretty fast. Playing both a DK and other classes helps. And yes, to the poster who spammed DK's are OP, thats what happens when you have a new class and don't understand their mechanics. Learn a Dk, play one for awhile, and figure out how to beat them. You'll start enjoying it a little more when you do that.