“It’s like a family.”
There’s that specific sense of community that no other online interaction provides. Nowhere do you get together with a bunch of people with both the purpose of achieving something and the purpose of having a good time, all in one activity. And then you actually go and do what you planned and accomplish that goal together.
Hell, I just spent almost my whole day with a friend. When I got back home, one of the first things I did was wish my corpies (other members of the corporation) a merry christmas. Even when I’m not playing the game I’m still in touch with them all, because they’re nice people and (even though I only just joined) feel like a group of close friends. It’s a lovely experience, really.
All that provided you play an MMO like it’s supposed to be played: with a group, be it a guild or just some pals. Because you don’t play an MMO to play an MMO.