User created content

User created content and game customisation are definitely features which set one game a part from another. Role playing games have been allowing players to create their game character for many a year but now some games are allowing even more in depth control. Items, environments, weather conditions even game mechanics are opening up new ways to interact in a game world and allowing new experiences from old ones. So how far could this go and when should developers say this level of control is too much? What sorts of ways can we allow users to have their impact on a game? Just things to think about I would love your comments on this topic.