When the Death Knight was announced as a hero class, I was quite surprised since it was something I didn't think they would do. But now, in a way I'm quite disappointed that only one new class would be added to the game. The game has been with us since late 2004 and until now the only new addition to the class selections is still on its way. I was actually expecting a new class when TBC was announced to come out. instead they just made shamans and paladins available to the opposite faction. I know it's a different situation but when the Diablo 2: Lord of Destruction expansion came out, it came with 2 new classes to choose from. I was hoping WoTLK would come with another hero class aside form the DK. But of course, I understand the situation. CM Nethaera explains below.
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. source
I think the plan is to have one hero class released per expansion from this point on although that may not be followed if it doesn't turn out to be what the designers want. But even at that pace, one per expansion seems a bit slow. Of course, balance is the key and the more classes there are, the harder it is to balance all of them.