Some video I've seen of a DK in action (on official realms, official alpha), definately do seem to indicate that DK gameplay is awesome. I know it's hard if not impossible to tell from just looking at gameplay footage, but as a longtime WoW vetran, something about the footage seemed.. 'right'.
It's probably the spell animations. They look over the top cool, with the DK himself conjuring massive explosions and stuff which look just so much more 'epic' than anything other classes can do.
I really hope Blizz does something for the other classes to make them appear as fun/cool to play, otherwise this sure as hell is gonna turn into World of DeathKnightCraft.