Megan makes an astute observation on the relationship between commenters and bloggers. I agree that commenters in some part reflect the persona of the blogger, if only because commenters often read and become invested in a particular website because it resonates with them or they identify with the blogger or the blogger's community. But Lizardbreath's remark that commenters not only reflect the blogger, but manifest more fanatical and concentrated versions of the blogger's views, is even more accurate.
I'd extend Lizardbreath's theory of commenter extremism to co-bloggers. Generally, a blog with a preexisting spin will become more radical as additional co-bloggers join and (especially) when original bloggers are replaced. I have noticed that this is often accompanied by a mysterious obsession with putting images in posts. Cases in point: Pandagon and Feministe. As time goes on and the original bloggers are supplemented or replaced, writing quality declines, the ideological or political content becomes less moderate, and people start using the image tag as a substitute for argument.
I think this has something to do with blogs being almost innately a niche medium. When you begin blogging, you try to appeal to a wide audience. But as the readership develops, the blog often finds its niche in the blogosphere: something is a feminist blog, or a progressive blog, or a techie blog. The most extreme, intentionally provocative posts gain the most links and the largest influxes of commenters, and the blog becomes part of what we used to call a webring based on the alignment of these frequently-linked posts. The new commenters, who were attracted by more radical posts, comment on other posts. The other blogs in the ring may continue to link, feeding more commenters who are already readers of these similarly-aligned sites.
Eventually the culture of the comments becomes aligned with the most extreme views held by the blogger. Radicalized commenters are typically the most enthusiastic participants in such a blog comment community (and the ones most likely to be asked to be co-bloggers due to their sizable contributions). The echo chamber continues to be constructed. Dialects and inside jokes develop, raising the entry costs for new commenters. Without fresh blood, intra-webring incest means that commenters eventually become twisted and horrible reflections of the original blogger.
Now what this says about me and my commenters, I can only speculate.