It seems a complaint I've heard a lot about Blizzard lately is that 2 years between expansions is just far too long. We'll languish too much without new content, and people will leave for Age of Conan and Warhammer Online and other games, they say.
Myself, I think the length between expansions is acceptable, provided that Blizzard is working on improving and adding new content. To some extent, they are doing this. 2.4 was a tour de force that granted us a whole new area to grind and quest in, and if Blizzard can be out patches like 2.4 on a regular basis, I can forgive them for a few delays in the expansions.
That said, I do feel like they could stand to pick up the pace.