I wish today's MMOGs lived up to the promise of player created content that was in the earliest online games. LambdaMOO set the pace. Pavel Curtis had a fanatical devotion to player-generated content. The whole world was player created. Players could build rooms and program new behaviours in MOO. Players even came up with their own government.
The first commercial MMOGs continued to let players shape the world. Ultima Online in particular had a significant amount of effort devoted to a player created economy and players building their own houses. But contemporary games like World of Warcraft and City of Heroes are essentially stateless worlds. Players can't make permanent changes. Players can't create a building, or modify a sign, or even leave a hat in the middle of the desert. And there's nothing at all like being able to write code to run in-game.
The one shining example of player created content in the commercial world now is Second Life. While not quite a mainstream MMOG, it's interesting for its devoted player-creators, impressive authoring tools, and successful player created games. Even the problems are interesting. Let's hope the business works out.
Player created content is hard. The gameplay and quality issues significant, it's hard to build server technology that scales when you don't know what the content will be, and it's hard to build good creation tools. But player created content is the interesting thing possible for an MMOG, both for the creators and the consumers.