Game designers have a term that I think is about to see a lot more use than it ever has before. The term is "stickiness"; and what it describes isn't nearly as gross as you might be thinking.
"Stickiness" is a property of multiplayer games; and in the past it's mainly been applied to MUDs and MMOs. The idea is this:
The more this can happen in your game - the more your player can, and is likely to, get involved with other players and become part of a community that they become invested in - the "stickier" your game is.
An important thing to realize about games is that any player WILL get bored with a game after playing it long enough. But if your game is sticky, people will continue coming back to it long after they normally would have lost interest in the core gameplay. They come back not for that gameplay, but because their friends expect them to be at a raid on time. Most gameplay becomes boring once you completely master it - unless you have a chance to demonstrate your mastery to people, especially people who matter to you such as your friends.
Almost none of the WoW players I know care much about the game's story (except for a couple very odd individuals I've met, who seem to know more about Warcraft lore than they know about their own lives). But they all care very deeply about certain very dramatic stories that happen in WoW. I'm talking about the oldest form of User-Generated Content: Guild Drama. WoW stories are the same everytime you play them, but the story of what's going on between your guildmates is new every morning...! It may not be high drama, but it's gripping because it's about people you care about, and it's really happening. (In my humble opinion, these are the only stories in MMOs that really matter at all.)
Stickiness is a big reason why the success of an MMO depends on reaching a critical mass of users. I have a theory that almost anything can, potentially, become the "core activity" that a community gathers around. In other words, it's possible for the most boring multiplayer game in the world to become the place where a community forms. (I imagine that many of the text MUDs and early MMOs would look dreadfully boring to us today, but they still became the home to vibrant and long-lasting communities. Ultima Online is still online to this day - the ultimate example of the legs that stickiness can give your game.) But you have to get a certain number of players to join, and to hang around, before you'll start to see this stickiness form; and to get that many players in the first place, your game must be appealing (so people notice it and try it) and have fun gameplay (so people hang around long enough to become part of a community).
This is key, and it's probably the single most important reason no one's been able to beat WoW. There have been some very sophisticated MMOs released in the years since WoW's launch; and while WoW has continued to improve itself and keep pace with its competition, nonetheless it might be true at this point that for each WoW player, there is some other MMO (or other game) out there that player would actually have more fun playing.
So why have these players not moved to those more-fun games? Because their friends haven't. This is why it's called "stickiness": if your community is part of one game, the law of social inertia (which I just made up!) dictates that those people are extremely unlikely to all simutaneously take the trouble to pull up their stakes, buy a new subscription, and all make the move into a new game, starting over in it as noobs. Though there are various barriers in place to cause this inertia, the strongest force is simply that people like staying where they're comfortable and sticking with what they know - unless something truly exciting galvanizes them out of their complacency. WoW is so hard to take down simply by virtue of the fact that they have so many players - most of those players are part of communities that they'd rather not leave. A game will have to be hugely appealing and fun to overcome the inertia of WoW players and convince them to shake up their comfortable communities.
Luckily, those who wan to unseat WoW need not abandon hope entirely! WoW has an Achille's Heel, and it's built into the very thing that makes them so successful. Tune in for part 2 of this series to find out what it is!
[UPDATE: Part two has been posted here!]