Well, there surely will be legal & copyright problems if the platform automatically replicates models and stuff when players upload it. Even if the player buys the model, he usually does not have the right to distribute it. There are certain provisions when it might work, for example the models that we've got allow us to modify and distribute them in our custom format that doesn't allow exporting it back to a well known format, to avoid users modifying it and releasing as their own (without license). In theory this could be used by the users too - if we release a one-way conversion tool, allowing users to only upload the converted models. Mind you, the license still needs to explicitly allow for the distribution.
Legal problems are to be expected anyway, so there will be a clause should put the responsibility on the users. I'm not sure to what extent this is possible though.
As for the quality and adequacy of the custom content - I assume there will be also custom realms, managed by people who acquire the realm (trying to achieve a certain goal in modeling the environment). So anyone making a game and allowing custom modifications & developments will be able to evaluate and filter the submissions, optionally adjusting some game-related parameters.
Say, someone submits a tuned truck: the developer may specify an in-game price, fuel consumption, power etc, to make it playing nicely with the game itself.