[Dire]

It's a matter of scale. Say you have a popular game with about a million active users. If you get something like an aggregate average of 1% of those users submitting an average of one mod per week. That's 10,000 mods per week to sort through! The first few months or so when it's hugely popular and everybody's trying it out, it wouldn't be suprising to end up with hundreds of thousands of mods to sort through.

Then you have to go through all the media for the level (textures/sounds/even geometric formations within the level) ensuring none of it is obscene beyond 'standards', etc, etc.. That's tens of thousands of man hours of work, per week, per game. And even if you could round up hundreds/thousands of volunteers, there's then the process of training them in what is acceptable and what's not / etc.

It's the same problem with XNA games. Anybody can develop games on your 360 if you purchase the developer's license (or whatever its called) on Live, but you can only share your games/software with others who have also purchased a license!

Guess we'll see what happens with UT3. Epic was/is really pushing for user created content and MS has alluded to working to allow it. Whether or not that turns out to be the case though...