Wow...lots of interesting perspectives here. An explanation (doesn't need to be more than a few words) of why the viewer thinks something is "excellent" is equally as helpful as why the viewer thinks the image needs work. If all you want to do is make the viewer feel good, you can simply instant message them and say "nice job." . I think comments should be more than "excellent" or more than "this sucks." Not a lot more, but a little more. It shows care on the part of the viewer to take the extra effort.