In any event, we have no plans to add another class in Cataclysm. We don’t want to saturate the game with new classes if they start to feel and play way too similar to existing classes. We came up with some unique class mechanics for death knights we ultimately think work very well. It’s a much younger class and took some times to really fine-tune, but we’re happy with the role they play in World of Warcraft now.

With two new races and a number of new race/class combinations available in Cataclysm, I have a feeling you’re not going to see newly created characters overwhelmingly death knights. (Source)