Usually, I review if the story left an impression. If I feel the need to say something (good or bad) then I do. For the most part, though, my reviews land on the positive side of things... not just "that was good, please write more", but specifically what I liked about the story, or the author's style. I will leave suggestions and what not if I feel the author really needs to hear it (and now that I think about it... don't we all need to hear it from time to time... hmm... may have to change my style.)
Anyway, I like to hear what people like about my stories (specifically!) and so that's what I leave for others.