World of Warcraft's Loyal Subscriber Base

Positive word of mouth is important for any entertainment product, whether it's a TV show, movie or a video game.  For an MMO, it's vital, because it keeps up the numbers of your audience and potentially expands it.  World of Warcraft has a leg up on any MMO in this regard, since it has 11.5 million evangelists for the game.

By their nature, MMOs are social and let you interact with other people.  World of Warcraft gives players the freedom to play as much or as little with other players as they like, but chances are players will team up with other users at least some of the time.  The game's social nature for some makes up for whatever perceived flaws it might otherwise have, and if you have an agreed upon way to hang out with you friends online, trying to switch things up is a hassle. 

World of Warcraft's large subscriber base provides Blizzard a huge advantage (along with hundreds of millions of dollars each month).  It skews new players towards WoW over some other MMO, since players are more likely to pick up a game where they know other people (practically guaranteed with World of Warcraft) rather than an MMO just starting out where players aren't likely to know anyone.  As for veteran players of World of Warcraft, having a high level character (or four) incentivizes them to stick with WoW, not to divert their time with something else.  It's a catch 22 for MMO developers that makes World of Warcraft very hard to compete with.