Girls gone WoW
Girls (or my friends and I, certainly) play WoW cos it has beautiful graphics, it introduces you properly with beginners areas, it has logical questlines to follow and in a lot of cases, our friends are playing. We go out and socialise together, guilds have become glue which bind people across countries (Dutch, Swedish, Norgwegian and Brits in our guild) and it's a way of being part of a team when you live in an isolated bit of the countryside. It's critical mass - all my mates play it and no other MMO, so of course I'm going to stay with WoW. It's where all my gear is, my understanding of MMO's is, my achievements are and my real life friends are.
It's the least geeky MMO, possibly, but it's also the most welcoming to people who've never MMO'd before.