Originally Posted by zahratustra 
Can't agree more Zloth! But how about player created content to extend long term interest in a game?
That definitely helps. City of Heroes did a nice job of that, though it was pretty rocky at first.
There is one big thing missing from player made content, though: it's not official. If the devs were to make a CoH mission where it was revealed that Sister Psyche was actually working for the Rikti it would be a spectacular plot twist. If you try it in your own player-made content, people are going to assume you don't know what you're doing. You pretty much need to make your own characters - and doing that can take quite a bit of time.