City of Heroes has become known for, it's the Mission Architect system. The player-generated content spans a huge number of adventures, ranging from the silly to the sublime. It's also robust enough in its search function that it's essentially added another venue of content into the game. The official site recently highlighted a feature in The Escapist, interviewing a number of community members and acclaimed users of the system about its ups, downs, and the overall experience.
The system has faced a few problems -- abuse of it early on led to bans and some content removal, since it's easy when creating content to be too generous with the rewards and too light with the difficulty. There's also the issue of inappropriate content, and of course the possibility that a large portion of what's available just isn't all that good. The full piece highlights some of the better arcs available (including more than a few Developer's Choice missions, which were selected as being good enough to qualify as official content), as well as the efforts and reasoning behind the more popular creators and what they've built. Matt Miller was quoted as saying that players did in one day with the system what it took the developers five years to accomplish. Quite an argument in favor of player-created content, that.