When you play a game on your Wii, you have to play it by yourself. When you play on PS3 or 360, you can see what your friends are playing and you can hop in and play together. When you play an MMO on your PC, you might become part of a niche group of friends who always play the same game.
I play a different game on Wii every time I fire it up. I only dream that I had a ps3 or xbox...but with Online pc games...I'm always playing the same stuff.
Two weeks ago, I got sucked into Allods. I was making new friends every night, I thought it would be great. And yeah, the game was very fun.
But then I DM'd another Neverwinter Nights game that Saturday (As I do every Saturday night), and suddenly my players got this renewed excitement about it.
The result: Now I have players instant messaging me on a daily basis. And they want me to DM pretty much every night.
Okay, Neverwinter Nights isn't exactly an MMO, but it has the main thread of an MMO that I'm pointing out here: Your friends list. I've been reading articles about how WoW keeps players playing using psychological tricks like intermittent rewards/drops...But I think even more compelling is the sense of community and the social pressure that goes on. Perhaps it is different for different people, but at least for me, I play games to have fun with friends. And if the people who are encouraging me the most are playing Neverwinter Nights...by golly, I'm playing Neverwinter Nights!