Tyranny of the Bloggers: It would come of no surprise to most members of the "blogosphere" that the vast majority of recognized, respected and (most importantly) read bloggers are of the Libertarian/Neo-Conservative variety. Much like talk radio, people of these persuasions are usually a) the most pissed off, b) the most entertaining, and c) the most verbose. Leftys tend to marches and demonstrations it seems more than the written word - perhaps it's the fresh air, perhaps the nifty slogans. In any event, the Left is significantly absent from the upper tier blogs - hell just about from any of the blogs you've ever heard of. Moreover, since most bloggers depend on links/referrals from other blogs to draw in readership (whether it's 10 people or 10000), the logic would dictate that if you want to get linked, you're going to have to be pleasing/agreeable to the site linking you. More often than not, therefore, it pays to be of a similar bent to your referral/link base. For example, Right Wing News isn't throwing people over to Krugman's latest on his website (to give an over-the-top possibility).
Yes, some of the "established" blogs are fairly apolitical: I'd argue Kaus doesn't really thump for anyone - he just likes to point out foibles and other fun gossip; likewise Instapundit is more a news commentary site, but for all the "rest of us" it pays to toe the line idealogically. You could say that this represents a more true representation of the state of mind of America, but again, I'm not so sure. I think the left-wing voices are more likely to be drowned out by the Randians who reproduce like rabbits. Probably the only way to rise among the fray is to have a non-blog source (say, your own article in Slate) that you can use to refer to your blog. To be clear, I'm not really complaining, just remarking - most of the libertarians are more interesting to read (shout out to Eno). The bloggers pride themselves on their grassroots ability to create discussion, and how they are different from "mainstream" media who are all "biased" (which way depends on who you're asking). I'm just not so sure.