It seems like anything like this should try to educate users about appropriate open-source licenses, and make license selection a highly-visible step in the upload process. It would be a shame if a community like this were plagued by the kinds of issues that eg. game mod communities suffer from due to unclear licensing status.
And they definitely need to make a clear statement about what rights the site gets and what rights the uploader retains.
"And they definitely need to make a clear statement about what rights the site gets and what rights the uploader retains."
Hi there, where would you like to read about this information? Perhaps a "About Licensing" page in the foot? Or simply bigger license badge on the sidebar of each project?
I think I'd like to see a Terms of Service link. GitHub's does a pretty good job of clearly explaining the situation.
Also, I don't think using or encouraging the use of a CC-Noncommercial license is a good idea, since it would prevent you or anyone else from offering 3d printing as a paid service to those who can't afford their own 3d printer.
And they definitely need to make a clear statement about what rights the site gets and what rights the uploader retains.