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. … 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.
The first step, therefore, to higher quality comments is “be more niche.” Discourage your marginal readers with technical language, obscure references, and lengthy posts. Your marginal readers are not of high value anyway, and driving them away is an excellent way to improve the average comment of your inframarginal readers.