The problem at the moment is that people want different things from comments. So sure, the facility for commenting exists, but how is the person who makes the comments meant to know what the artist is after? I tend not to suggest improvements any more because although I know 95% of comments would be well received, I don't want to offend someone who has just posted to show something they created and couldn't care less about colour balance or soft shadows or point of focus. If your daughter brings you something she painted at school it would be pretty despicable to start on about its faults, but if you went to an art gallery and commented "wow, that's great" for each painting, people would think you were a simpleton. A critique gallery would go a long way to solving this problem both from an artist's and commenting artist's perspective. It certainly would do no harm. And yes, criticism is both positive and negative. I personally have never left a purely negative comment. I've always balanced it with a positive, more than one if I know the person is a beginner - like me! :)