I found the following idea posted over at Renderosity. You might find some merit in it. It sounds helpful in anycase.
In reply to:
How about the following to solve or address the mediocre content problem?
Require downloaders to maintain a ratio of ratings to downloads, so that they have to rate at least 50% of the number of items they download.
Require uploaders to maintain a ratio of positive ratings and ratings period, so if they upload too much that isn't viewed and rated, or upload too many items with low ratings, they lose their upload priveledges or have to take down those unviewed or low rated items?
It is a miracle that curiosity survives formal education. - Albert Einstein