The social element, the rich and expandable storylines, and the relatively low hardware threshhold to play it. Internationalization helped a ton, as well. In China, it's a pop-culture phenomenon -- advertisements for other brands use WoW characters a lot.

And, of course, Blizzard/WoW has a sustainable business model.

I have to say, however, that the UI and some of the complexity related to tools and weapons is a bit of a turn-off, and is probably a barrier to more widespread subscriptions, IMHO.