Ownership is key. You have to give players a sense of ownership in the game. This is what will make them stay--it is a "barrier to departure." Social bonds are not enough, because good social bonds extend outside the game. Instead, it is context. If they can build their own buildings, build a character, own possessions, hold down a job, feel a sense of responsibility to something that cannot be removed from the game--then you have ownership.
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Update: 11 May, 2009 - Edited post, applied label, and removed broken link.