Here's the skinny, the lowdown, the 411 etc on the death knight choice.

First off, it fits in with Northrend extremely well for obvious reasons.

Second off, many people tend to have this strange belief that adding a new class is simple, easy as pie, and a cakewalk all on a Sunday afternoon, but it's not. There is a lot of work that goes into it and many considerations for how it will work, how it will work with other classes, how the lore ties in, how quests and npcs also can tie in to this new class, itemization, and a whole lot more. It takes a lot of forethought, testing, and care to add something this major to the game and for each new class that is added to the game, come the concerns about not maintaining the uniqueness of each class that already exists.

I'm not saying that the possibility of more classes being added in the future isn't a possibility. We had many ideas of what class we'd add in for Wrath of the Lich King prior to settling on the death knight. There are many more possibilities for classes we'd consider in the future as well, but we aren't going to rush to add them in and risk creating a situation where other classes lose their uniqueness and their value within the game.