Thinking about it more, I feel I've figured out where my distaste lies, but also what I do like. There's this tendency for people to try and "blog" every day, or even multiple times each day, even if they don't really have anything to say. It's the mental equivalent of whistling... not really useful, and often quite annoying to anyone else. I've never been a fan of quantity over quality, and blogs often become a shortcut for people, letting them spew their half-formed thoughts to the masses.
It used to be that, were you to want to share your thoughts, you spent a lot of time writing well formed essays or articles, and people who responded did likewise, quoting both you and other sources to enhance their argument. As "publication" no longer became necessary, the standards dropped, but even the time it takes to make a proper web page at least offered an author some chance for reflection and review of his thoughts. Now, between forums, instant messages, and blogs, we spend less and less time criticizing and improving our own ideas, relying on the knee-jerk reactions/comments of others to refine it for us. On top of that, it gives one no time to cool off from a possibly heated argument. We just write about things immediately, even if they won't seem important 5 hours (or 5 minutes) later. Blogs just seem like the latest step towards the ADD society.
Of course, these are all grievous over-generalizations. Many self-named "blogs" do have well thought out, article/essay quality writing. Likewise, garnering immediate feedback from others lends a certain honesty to the whole affair. I think my concern is not so much that blogs are becoming mainstream, but rather that they'll start to replace traditional, multiple-draft methods of writing. It doesn't bother me in the slightest to find programmers keeping daily blog posts about what bugs they're working on, or artists talking about their current project... watching a CNN newscast spend the better part of an hour debating the political opinions in the "blogosphere" (seriously, can we shoot the guy who thought up that word?) makes me want to hurl a brick through my television.