People have gone on and on about the negatives of user created content. Mainly in multiplayer games, people always have been conscious about user created innuendo such as porn related content in games. Porn has made its way into games such as second life, spore, and games that have been allowing user created content to their hearts desire, but here's where the developer should step in.
If the game developer didn't want porno in its games, then they should put in place a filter, ala game monitors that are chosen the same way forum moderators are chosen. No, not a popularity vote, but by submitting reports of negative content being created. By giving the real moderators eyes and ears via eager participating members of the community, they can root out all that is blemishing their great work.
Now, ask yourself this. When is the last time you played a really great MMO or Online game that had user created content made up on the spot inside the game made by players of that community?
This is a niche that gamers should ask for, and it should be ripe for developers to start creating. And isn't it about damn time?
Edit: yes, this is the reason spore is going to be so popular.