So are people getting cease and desist letters for the content of their galleries. Or has Renderosity just gotten paranoid?

Fair use includes, criticism, comment, news reporting, teaching, scholarship or research.

So for example include a comment with Fan Art like, "This is my interpretation of a scene from Star Trek."

It's a form of Free Speech and should be defended and fought for. If Renderosity will not support or defend that right, Post it somewhere else.