Eli Dourado writes

The commenting dynamic is very different on small blogs than it is on popular blogs. On small blogs, people typically comment when they have something to contribute or ask that is relevant to the post. These are frequently of high quality (relatively speaking; recall Sturgeon’s Law: 90 percent of everything is crap). On more popular blogs, this positive commenting dynamic is confounded by the presence of eyeballs. Every post is read by many thousands of people. For the self-involved who could never attract such a large audience on their own, this is an irresistible forum for expounding pet hypotheses, axe-grinding, and generally shouting at or expressing meaningless agreement with the celebrity post-authors.

which sounds plausible but it assumes a sorting condition that is debatable.  Presumably everyone, the high-quality and low-quality commenters alike, want a bigger audience.  The question is one of relative elasticities:  does a percentage increase in audience lead to a larger percentage increase in low-quality comments relative to high-quality?  I am not sure.

There is a perception cloud because most of us think that our own comments are the highest-quality ones and so the overall pool of comments appears distorted downward in quality.  But there is one force for Gresham’s law that may swamp any having to do with commenters’ motives.  There are simply more ways to be wrong, off-topic, and crazy than there are to be right, on-topic, and smart.  So even if the sample scales proportionally with audience size, the space of good quality comments will be exhausted quickly leaving the long tail for the riffraff.

Bottom line:  the way to maximize the proportion of good-quality comments is for the bloggers themselves to use up all the crazy ideas at the start.  I like to think Sandeep and I are getting better and better at this.