Toeo of Dragonmaw wrote in to ask about a clarification on Hero classes (of which the Death Knight will be the first). What are they, what will they require, and what does it mean to be a Hero class? We are nothing if not helpful, so here's what we know about Hero classes (mostly from BlizzCon).
There were a number of ideas floating around about how Blizzard might implement Hero classes (mostly from D&D, in which Prestige classes are the precursor), but what they landed on was an "unlockable class," that's opened up to your account via a quest with a level 80 character-- that quest hasn't been created yet, so we're not sure what it will require*. So, to play as a Death Knight, you get a character of any class to level 80, do the Death Knight quest, and once your character has completed the quest, you can log on, and create a new character (of any race, we're told) that is of the Death Knight class.
That new character then starts at a higher level (anywhere from 55-70, but Blizzard hasn't decided where yet), with starter equipment of the appropriate level (again, Blizzard hasn't decided exactly which equipment yet or how it will work), and then the Death Knight character can work their way up to level 80 as well as a completely different character. Death Knights will have their own Rune resource system, and they will have their own three talent trees, just like all the other classes. It is a completely separate character, unlocked and able to be created when one of your level 80 characters finishes the quest.
*Update: Commenter Avalanche makes a good point: we don't know much about the quest, but we know it will be similar to the Warlock epic mount quest, and that it will not require raiding. Thanks, A.!