99% of all user-generated content is useless. Not because it sucks: bad content can still be useful content. But if you wander into SecondLife, you'll see acres of completely worthless buildings and billboards - ugly, uninteresting, worthless. Even the nice stuff will rarely hold anyone's attention for long.
There is a reason for this, of course. I'll want your advice after this essay, so soak this up:
The content's only purpose is social.
In most games with user-generated content, the only real effect it can have is a social one: a new backdrop, a new animation, a new texture. In 3D chats, these are literally just frosting: a new skin on the same beast. In SecondLife and similar, you can script your stuff to have world effects - such as you'd do with a gun or a car. However, since the world is basically consequence free, these are still only social tools.
Sure, these games allow you to buy and sell for real money. But what has value is determined by what the players most want in the game, and all they can want is fluff. Luxury items. The game rules don't really allow for anything that is actually valuable, because anyone can do anything. Artificial limits and imaginary prices are the name of the game.
So, a possible solution is to make it so that content can/should/accidentally will accomplish something in the game world, give the other players something to do. For example, if you design a building to be a generator, not only do you get some kind of benefit (the ability to recharge your stuff, probably), but your building can be found replicated by NPCs all over the place, ripe for raids and role play.
There are two problems with this kind of idea. The first is one of algorithmic limits: the design variation between generator buildings is likely to be fairly minor. The more plausible variations you allow (aesthetic, material, design), the more intricate your algorithm has to be. At some point you start to say, "we'll let the players upload aesthetics, materials, and design parameters..." which is cool, except that it's about ten years of programming to get it to work.
So you end up with a bunch of content that's basically the same except for relatively minor topological differences.
The other problem is that you'll probably run out of things to accomplish. How long can a player stay entertained just building bigger and more refined versions of things that they already got working?
I suppose that could be somewhat fixed by making for a huge variety of situations - IE, move on to the next planet, which has a very different environment, or fight the next boss, who is basically immune to whatever you are currently geared for. It could also be fixed by making the world PvP, but I don't suggest it.
It also might be fixable by making the specifications about stories and mystery rather than statistics and energy. Tough, though.
Anyhow, if anyone has anything to say on the subject, or can figure out any way around any of the problems, post a message. I'm kinda spinning my wheels.
On the plus side, the new DosBox plays Quest for Glory 4 perfectly. :D