On the other side of the coin, I don't comment unless something really blows me away ... and when that is the case I usually end up saying something like "OMG that is amazing!"  If I post a render I don't expect comments, I just post because I enjoy it.

What turns my head in an image?  Great attempts at realism get my attention every time.  Great use of postwork that shows an artist's individuality.  Dramatic lighting.  Things that are well done and out of the ordinary. 

And ... I can understand how sometimes criticism can be harsh ... but if someone offers a suggestion they are doing it to help you improve. There is a difference between someone saying "That render is pure crap" and "You know, next time you might want to add an extra light behind the character to pull them away from the background." One is insulting, and the other is helpful.

Without constructive criticism and suggestions, I think it's pretty tough to grow as an artist. 8-)