Lots of people have touched on my reasons.

People: I've made gaming friends in other games but the only ones I keep up with long term are wow players. Half my online social interaction is chatting with gaming friends and I miss it when I am taking a break.

Multiboxing: Wow is just the best multiboxing experience. In other games you have to fight the interface and the game design to get it done. In wow the addon api is so flexible most things that annoy me can be fixed with an addon.

Goal setting/persistent progress: there are so many things to do, you can switch from mini game to mini game when you are tired of actually pveing (as I am right now).

great AH system: (when using addons) I try to get into gold making in other games but some are just terrible (rift, with their new enormous deposit policy, wildstar where the vendor materials for even early patterns cost the equivalent of a quest worth of silver on top of the gathered materials).