Don't get me wrong, I'm totally glad we're out of Northrend and its utter dedication to making Death Knights look like the Be-All-End-All Class of WoW. But you have to admit, it was a very Story-driven Class, moreso than any other single Class, and that Story came to a screeching halt the moment Arthas died.
While some might argue "Thassarian and Koltira in Western Plaguelands blah blah blah", the fact is, we're left with a pretty strange picture. Darion Mograine said Death Knights would have no place in the world of the living once Arthas was defeated. He was also supposed to reclaim the Ashbringer, which obviously never came to fruition. Thassarian supposedly cut all tied to the Alliance in Northrend, so is he in Western Plaguelands by choice, or did Blizzard kind of forget his previous story-arc?
Mists of Pandaria will likely be "the Monk Expansion", just like Wrath was "the Death Knight Expansion". But considering Death Knights still exist in the game, it couldn't hurt to at least offer some kind of reasoning to their continued presence in the Lore, could it?