Really

Current virtual worlds take different approaches to user-generated content. The more controlled environments give the users almost no way to alter the world, while more open platforms give users almost complete control of their surroundings. Allowing user-generated content is attractive, because, hey, free content development! Of course, there are risks also, like copyright infringement, “mature” content, and teh ugly.

However, even when user-created content is allowed, many of the current and emerging virtual worlds require users to create 3D models in external applications, and then import the models into the world. A sensible approach, reusing existing tools, and not trying to reinvent the 3D modeling wheel.
The risk is that it leaves the users with little or no way to modify the environment directly. The problem is nicely illustrated in a post on Journeys with Jayce, describing how to be an effective raid leader in World of Warcraft:

For some reason, we could not manage to keep the mob in the middle. So I decided to use a player corpse, positioning it right smack in the middle of the pit, and using that as a marker for where to place the mob. The goal was to make that drake die and land right on top of the corpse. We never had a problem with positioning after that, and it worked so well, I ended up dragging along a dead gnome all the way up to the back of NToV, and using him as our mob marker for all the nameds.

Being able to change objects in the world and move them around, enables users to develop their own collaboration tools as they need them, when they need them. Activities like highlighting, voting, and brainstorming, can all be accomplished by moving and altering objects — No further application support is necessary. Of course, built-in support for those activities might be much better than what users can come up with on the spur-of-the-moment (”stack your dead gnome next to the proposal you want to vote for!”), but it will be difficult to come up with all possible tools that users might need.

Voting by manually stacking blocks (no gnomes were available)

If your virtual world only allows you to talk to other users and interact with the environment in pre-defined ways, you lose much of the benefit of collaborating in a 3D environment. By providing tools for changing the environment directly, even if it’s just dead gnomes, virtual worlds enable users to interact with each other in new ways.