[Tyrannosaurs]

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.

[startuprules]

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)