I am completely enamored with the DK. I have played all the major MMOs and leveled a toon to max in most of them. I would still say that my EQ2 Brigand was my favorite class ever, but this is followed closely by the DK. (and the EQ1 bard in there somewhere) The DK truly feels Heroic. I think the rune mechanics and the longer skill rotations are fun. It is the type of class that anyone can do well with it, but leet players with finesse and forward thinking can really make this class standout among their peers. I love that in a class.
There are so many toys that give a real "fun factor" like: running on your mount over water, extreme mount speed, a group run speed aura, a perma pet (if wanted), crazy AoE fun, self heals, a grappling hook, a dark-look, and of course platemail+2handers topped off with creepy spell animations.
I look forward to making this toon into an ubertank. In the game of WoW I have found that I am best in a tanking role and enjoy it the most. I am bummed I ditched my Paladin but this DK has more then filled the void.