I also think a RP is more successful when the characters and the RP itself really comes together as one.  When a player feels the character has connected to the roleplay, and takes an interest in the potential consequences (whether good or bad), I think they're more likely to want to keep going.

Of course, you'll need a good community.  It's great to see player-characters actively engaged with other player-characters.  And don't forgot a good GM - and that is something I am not.