World of Warcraft has a couple of other things going for it though.
1) Network effects. If you've got friends who play WoW then that's a major reason for you to play it even if there is something better out there. Of course you could all move to the better thing but they won't because...
2) "Loyalty". Anything where people invest time which can not be transferred (for instance in levelling up characters) tends to incur loyalty, if only because they don't want to start over.
Agreed. Also, there is the value perception. When players have to sort through all these MMO games to play, the paid ones stand out over the free ones. (for those players with limited time to play, and can afford to pay)
WoW also offers a lot of free months to players. Sign up a friend, both of you get free months.
I almost said "and WoW isn't the best damn product," but that depends on your definition of 'best.' It almost mirrors the Mac/Windows thing. Connoisseurs tend to scoff their noses, yet they just keep on selling...
Maybe 'not innovative' would make more sense. WoW is a great game, and I have a ... long and very personal history with it. But it's a Blizzard game; their MO is to make the most average game possible, but make it incredibly polished. Many of my fondest memories are of Blizzard games, but (as of late) they're rarely innovative.
WoW is the gold standard of fantasy MMOs. It just doesn't do anything new or interesting; it only does a small set of things, but does them very, very well.
1) Network effects. If you've got friends who play WoW then that's a major reason for you to play it even if there is something better out there. Of course you could all move to the better thing but they won't because...
2) "Loyalty". Anything where people invest time which can not be transferred (for instance in levelling up characters) tends to incur loyalty, if only because they don't want to start over.