LK added enough new areas/content at high level to make people want to play the new areas multiple times (Storm peaks was so much fun to level through). Then it added a serious raid challenge - the only people who got kingslayer on their first attempt paid a lot of money to be carried through the fight.
So far cataclysm has forced people to sit through starter areas again and teased with raid content. Furthermore is it just me - or is WoW underwater NOT fun? I hated quests which forced me to dive in vanilla, TBC and WOTLK - now the start of the expansion is underwater? FU!