One of my all-time favorite posts at Marginal Revolution is on the quality deterioration of blog comments, where TC guesstimates that roughly the first 15 comments on a post are the best, after which quality takes a big downturn. Since reading this post, I have developed the habit of trying to skim the comments to see when comments "turn south."
Based on my experience, here is my ranking of the fastest deterioration of comment quality among the blogs I read. Note that I do not think bad comments are a reflection on the blog authors:
1. The Chronicle of Higher Education: By far the fastest depreciation of comments, which seem to almost always be from professors. Case in point, look at the comments on this post for a story on students with unprofessional email handles.
2. Freakonomics: I believe this is just because they are on the NYTimes website.
3. Megan McArdle's blog: However, this has drastically improved since she started moderating the comments. For whatever reason, she seems to draw a lot of venom from a fair portion of readers. Surprising to me given how good her posts usually are.