Because I don't. This is a question I've been meaning to ask for a while, finally got around to it. So long as it's really their honest opinion (and not just stupid "flaming"), and they clearly and logically explain the reasons behind their ranking, and maybe even offer a few words of advice on how to fix things. I personally am a member of Renderosity first and foremost to learn more about the programs I am using and about art in general. Particularly if I am a beginner at a program, I do not expect 20 rankings of "excellent"; nor do I have a right to, beginner's luck aside. Personally however, to avoid discouraging other artists, I avoid putting less than "Great" on an image. If I really can't in good conscience rank an image "Great", then I just won't rank at all. =)