I've seen some excellent DKs, skilled players who take the time to actually learn the mechanics of their class. But for the most part people pick up a DK because it's an easy level 55 alt, and they just run around using their abilities haphazardly, with little idea how things actually work. Like trying to tank alongside a DPS DK with Frost Presence up ("But it gives me more HP!") or those fellows who run around Death Gripping everything, without understanding that it is, in fact, a taunt that FORCES the mob to attack them for a time ("Oh my god, you're a bad tank, why is it on me!!").

Sigh.