I'll share my own subjective observation - I feel that the overall "quality" of comments has remained high, at least in terms of writing quality and persuading arguments. That is to say, I don't often see many "dumb" comments ranked highly.

However, I have noticed lately that the highest ranked comments are almost always those which vehemently disagree with the author of the original post. I seem to see fewer comments that begin with "The author makes some great points, but he may not have considered..." or "This was a great project, the author should consider expanding it by trying..." More often, I see comments that are cynical and dismissive of the author's points. While a healthy dose of cynicism is necessary, and I love hearing both sides of the story, some of these comments definitely lean a little closer to the "mean" side of the spectrum.

I suspect that this is a subtle side-effect of not being able to see vote counts, that somehow results in fewer votes being cast by those with neutral or positive opinions, but I'm not quite sure why.