I fall in the same category as scattercat. When I first started to comment, my comments were short, partly because I was catching up on backlog, and partly because I was not aware of the community at that point. As time went on, I commented more often, and gave explanations as to why I felt a certain way about a story, because otherwise, I don't see the point. Even when I love a story, if I don't have anything to add, I often don't comment, because it doesn't add anything to the conversation. If I do post saying what others have said, I try to alter things in a way to add at least a little something, or express my take on a certain aspect that I liked or disliked.
Finally, I think that meh comments, without explanation, can seem more like an attack on the author than the story, and that may be why they are seen in a more negative light. I have posted some "I got nothing out of this story" comments, but I try to follow that up with why, e.g. "I didn't get much out of Black Swan because the main character was an abused young woman who was a ballerina, none of which I am."
