deviantArt has a user selectable level of critique option for each image they post. You can set the level of critique desired and a notice will be displayed with the image describing the kind of critique you're looking for. It runs from:

Comments Disabled - no explanation needed.

Critique Discouraged - meaning you prefer not to receive critique-heavy comments. In other words, you prefer ego-stroking comments like "wonderful", "excellent", and other such comments.

Critique Welcome - meaning you feel indifferent about the intensity of critiques that a viewer may include in a comment. You don't discourage constructive criticism or fluff. 

Advanced Critique Encouraged - meaning you openly invite all manner of advanced critique on your image. This is especially useful if you are actively working on improvements for a work of art. Fire away, you can take truthful, constructive criticism.

This is a scheme that Renderosity might consider at some point in the future. In the meantime, you can always leave a similar notice in the notes field when you upload an image. No, it probably won't stop the trolls, but it will give the general viewing public an idea of what you look for in a comment, and it might even encourage people to leave a comment. As it is now, many people are a bit fearful of leaving a constructive comment because they don't know how it will be taken.

It's all fun and games...
Until the flying monkeys attack!!!