Cataclysm. It took away everything I disliked about the game up until that point and made the level experience way more fun and focused for me. Having to run back and forth between zones looking for quests to do and having to grind levels because of badly done quest progressions never struck me as particularly fun, just annoying busywork.
I was never a raider or PVP player though. I played more for the world and the atmosphere than the gameplay, so when the gameplay finally began to catch up, I was pretty much in throes of bliss. Haven't played Pandaria, so can't comment there.