The only expansion I had to quit was Cataclysm. World Revamp did not deliver, end game content was barren, literally no plot other than "evil dragon broke the world, please fix".
I know WoD should get the vote but I stayed throughout the expansion and its content and as horribly boring and lonely it ended up being I had an alright time making gold, alts and finishing rep grinds. Not as great as MoP was but satisfactory whereas Cataclysm has nothing going for it. End game zones split up so far apart you spent more time flying to Hyjal than it did to do the dailies and when you did your 15 for the day, you had nothing else to do.