If I really like an image I'll comment, if I think it's garbage or it's just not my taste then I'll just move on.
Constructive criticism is a good thing, but I think this differs from personal opinion. If people are saying they think the colour tone was wrong, the camera angle is wrong, the tree would look better over there, I don't like the outfit, to me this is an opinion, it doesn't mean the artist's work needs correcting. If they could be given comments to help them technically with their 3D app settings (ie render, lights etc), postwork, or specific help to what they have asked for, then that's fine. But to say you think such & such could be improved just because it's more to your taste, I find is not very helpful unless it gives the artist new ideas to think about. 
If I was to post a fantasy image & someone stated a particular thing didn't look realistic I'd pull my hair out!