I know what you mean, but don't think a tick box would help... Too many members, too many opinions. Renderosity already needs some order (just check the new opinion poll hehe). I just don't think you can compare 'rosity to a high-end gallery, I mean it's good, than's why there're so many members and it's a great place to learn.

Maybe people can just mention on their images that constructive criticism is what they would prefer.

Just my 2 cents :)