Also, no-one seems to have mentioned this, they're a DPS class, like every other class in the game can be, so if you're going DK, don't expect to be in huge demand in groups.
Warriors, Paladins and Druids can tank better than DKs.
Priests, Paladins, Druids and Shamans can all heal. (ignoring the small self heals DKs have.)
As for DPS, I'm not sure how Rogue, Mage, Hunter and Warlock DPS stacks up against them, but at least they all have some crowd control abilities, I'm not sure what the DK is bringing to the group apart from 'me hit fings hard'.
I'm no expert, as the above probably shows, I have one of each class on the go, mostly between 60-70.