Am I the only WoW player in the existence who doesn't want a Death Knight? Sure, they seem totally hawt an' all, but if you have to sacrifice your old character for it, instead of it turning into the Death Knight or lvling a new character from 1, I'm not intrested. I love my characters, and would never change them for some new, bright an' shiny character handed to me for doing some quest line. 
No, its been confirmed already that you don't have to get rid of your main character, you're just unlocking the class for play is all.
theres gonna be a hero class for each character, death knight i think is for warrior/paladin, can't remember/care much less.
So shamans will get one too, i doubt blizz will not let you go back to your original character, but i think hero class is going to be the way forward, wheather its PVP focused or PVE attuneable, i don't know. But i forsee everybody working to 80 and getting Hero class on the way.
Well, I don't know if there is going to be an every class-specific hero class, or if we'll even see more at all. Sure Blizzard's said there's a possibility for more hero classes to be added into the game, but I don't think it'll be one for each of the different eight classes, I mean, look at WC3, there were more than eight hero classes. Death Knight, Mountain King, Archmage, Blademaster, Far Seer, and a few others I can name, but not off the top of my head at the moment, I could if I had my copy of WC3, but my friend whom I loaned it to lose it.
But on the subject of Death Knights, I don't think they're going to be all that great as they're glorified to be. Sure, dual-wielding or two-handed tanking seems godly powerful, but would it honestly be the best and every raid would die to have? If so, it would totally fuck up the itemization for other classes and the Death Knight class itself, as with most of the tank classes (warriors excepted), there's a flaw in each of their tank talents. Paladins' mitigation and health, a druid's not being able to block and some mitigation factors and all that nice stuff, so Death Knights wouldn't see that many tanking roles, prolly a 2-handed rogue who can dish out enough dps as if it were dual-wielding with a 2-handed weapon. Warriors are the best tanking class there is and no other class should surpass them in tanking, but by all means let the Death Knights be a hybrid tank/dps class, I can't rightfully bitch about hybrid tank classes, my paladin's protection spec as it is.