The first problem I see? Remember how many Blood Elves and Draenei there were on the servers after the last expansion? That's not so bad with a new race, but with a new class, the whole balance of gameplay might be thrown off. Not to mention that Feral Druids and Prot Paladins might not be real happy with even fewer tanking roles.
Still, I'd play one, of course. Truthfully, I think this is all kind of a moot point-- if the Death Knight class is real, I hardly think it's the only hero class that is going to drop with a new expansion. Even if the Death Knight is the only class they reveal this weekend (remember how long we had to wait to learn about the Draenei?), my guess is that there'll be more announcements to come. Twenty four hours left!