Blog factionalization - The ecosystem of blogs is increasingly balkanized. It's hard work to push against your own ideological inclinations, and most people don't do it very often, especially after writing a blog for a long time. In addition, there are substantial incentives to develop a consistent brand to encourage repeat visitors, and the way to develop a brand is usually to deliver opinion from a consistent ideological perspective, which draws visits from readers and links from bloggers who share those views.
Over time, we see increasing clustering by ideology, with Kos, Marshall, Atrios, etc. in one network and Sullivan, Reynolds, Powerline, Belmont Club, The Corner, etc. in another. Of course, everyone filters the information they receive, accepting some points and discarding others; the problem is when like-minded communities start mostly talking to each other. The result is that their views usually become more extreme. This is what Cass Sunstein calls the law of group polarization in his book Republic.com, and while things haven't become nearly as bad as he predicted, the phenomenon is clearly at work in communities of right- and left-wing blogs that constantly feed each other horror stories about their opponents that are taken as representative.
For example, Reynolds is constantly being sent -- and linking to -- negative posts and articles about the left, and posts very little counter-stereotypical material. Similarly, he freely admits that he filters out bad news from Iraq since he believes the media already covers it.
The incentives for bad polemics - As Matthew Yglesias wrote in a great post last year, writers who make inflammatory, misleading or wrongheaded arguments attract vastly more controversy and attention than those who make good ones. Bad arguments generate many responses from the other side (Yglesias cites Michael Moore and Samuel Huntington as examples of this), while good ones are strategically ignored. Andrew Sullivan, for example, can be a brilliant writer (see his early work on gay marriage), but his blog writing gets so much attention in large part because it's both interesting and almost totally unfiltered, which leads to lots of mistakes, retractions and so forth. A "love him or hate him" personality cult arises, and the audience swells. Reynolds writes quick, cutting posts as well, and this has clearly boosted his audience for the same reasons. Further down the blog food chain, a similar dynamic applies -- ridiculous "Fiskings" and other lousy polemics are often the best way to generate readership and links from top bloggers.
I still believe in the potential of blogs - as we wrote in All the President's Spin, the blog culture of fact-checking can help correct inaccuracies in the mainstream media. But this depends on (a) blogs doing credible fact-checking and (b) careful filtering of it to sort the actual mistakes from the ideological garbage. If blogs are mostly peddling nonsense, they'll get tuned out.
(Notes: In the interest of disclosure, Glenn was a good friend to Spinsanity; I don't mean to pick on him in particular. Also, as a political scientist, I have a real appreciation of the importance of ideology in bringing coherence to politics, but it has obvious and important downsides as well.)