Primary: suspension of disbelief is better in WoW than in any other MMO I've played. Just everything about WoW is well polished and fluid. Keeps me focused on the actual playing of the game rather than working around glitches, laughing at graphics bugs, etc.

Secondary: Configurability. The plethora of addons and interface config options means I can set it up to play the way I want to play, which again keeps me focused on gameplay rather than fighting interface problems or grumbling (get-off-my-lawn style) about how terrible some of the UI behaves.

Frankly, despite content being old, it's just a better game than anything else I've played. Others have some decent novelty pull but that wears off after 2-6 months.