I, for one, comment on works that inspire me to say something. I've only once given polite, but negative, criticism because I wanted to see the piece improved. It was a great work, but had one very glaring problem that I thought needed to be addressed in it's text. The unfortunate result though was the person pulled the picture, greatly dismaying me because I did like to look at it. Usually my comments are encouragment, suggestions or questions for that artist though. I also see a LOT of images that I like but I don't comment on because I don't feel moved to say anything. At least I increase the view-count on the image even if I don't say anything. IMHO