Too many participants
Popular blogs have thousands of readers and hundreds of commenters. The individual or team that authors one of these blogs will typically post at least one new essay a day. The readers of the blog receive rapid notification of the new post through mechanisms like RSS feeds. This often causes a feeding frenzy effect for many, many comments are made in a short period of time. The large number of participants in the discussion makes it difficult for a serious, in-depth discussion. It is impossible for the author(s) to respond to each commenter due to their number. Many choose to respond rarely or not at all. The large number of commenters results in many diverse side topics being introduced. The number of participants also discourages long responses but rather encourages short, witty comments in order for the commenter to attract the attention of fellow readers. I imagine that it sometimes resembles salon discussions when the number of participants grows too large. There ends up being a lot of humorous remarks and laughter, but not much serious discourse.

Rapidity of discourse
With at least one new essay a day being posted on the blog, any single topic has a short lifetime. Even if the same topic is continued by the authors, it rarely extends beyond a few days. This encourages quick responses (as does the previous item) which is antithetical to serious discourse. A serious discourse requires thought and reflection. Facts need to be gathered, arguments sharpened, and assumptions challenged. A short time line makes this near impossible. Instead, a popular blog community can appear like a group of young children who become bored with a game after an hour and move on to the next one and then the next one and so on.

Lack of authority
While the authors of a popular blog often (sometimes?) have some claim to authority in the field of interest, commenters usually don't. Nothing is required to comment on most blogs beyond Internet access and an email address. There is no filter process to prevent the uninformed from taking an active role in the conversation. It is possible for this to be a sort of benefit if everyone would check the veracity of presented claims, but that is quite unlikely. The commenters also participate on a level playing field that has its advantages and disadvantages. The disadvantage being that an authority is not guaranteed of any more notice in the discussion than a junior high student. While the current system for publishing books and journals is far from perfect, it does perform some needed filtering of the noise of the masses.

There are positive things to say about blogs as a discourse medium. They present a vast opportunity for exposure to new ideas and information. These ideas and perspectives can force you to think. I should also mention the low cost for access. For all their good, I don't think we are warranted in claiming any great increase in serious discourse resulting from blogs.