First half of MoP was daily hell. After ToT released though, expansion picked up some serious steam and was pretty good.
Say what you will about it being "Kung Fu Panda" expansion though. Regardless of you liking it or not, Blizzard did nail the aesthetic. Story was pretty solid too overall. SoO was a bit drawn out, but overall this was up there with Wrath for quantity and quality of content. Especially at the end.
Cata and WoD were the opposite, came out strong, decent initial bit of content and then it fell flat. Cata was around mid Firelands, and WoD around Blackrock Foundry's release. They just had no momentum.