I think the reason many people play MMO's goes well beyond game mechanics, or even social interaction (though that is an important part). With MMOs, we're introduced to worlds - virtual as they are - that provide something that has long been extinct in the "real world": genuine reward for genuine effort. In MMOs, you work hard at your objective, and earn the reward. Every time. Without exception. Work hard enough, and you'll eventually get what you're after, whether it be a piece of gear or reputation or whatever floats your boat. In the real world, that's not the case. You can work your brains out, and someone is always there to prevent you from fully harvesting the fruits of your labor. You may work harder than someone else, but that someone else may get the promotion due to affirmative action. You work hard to make money to support yourself and your family, but the government reaches into your wallet and takes a sizeable portion of it. MMOs, generally speaking, are just. There are no special exceptions. Nobody in your way. Nobody taking what you've earned without your permission. You can build your character, his/her career, his/her life, through the sheer power of your own will and effort, and not get screwed.

MMOs are a place where your individual effort matters. People want that in their lives. It's just kind of sad that MMOs are the only place the majority of people can find that these days.