Reader JK sends along a link (abstract here; full pdf here) to a study of the differences between the left and right blogosphere. Their main findings:
Sites on the left adopt more participatory technical platforms; are comprised of significantly fewer sole-authored sites; include user blogs; maintain more fluid boundaries between secondary and primary content; include longer narrative and discussion posts; and (among the top half of the blogs in our sample) more often use blogs as platforms for mobilization as well as discursive production.
The study was done in 2008 and I suspect that there is a bit more mobilization and calls to action on the right now than there was then, so the differences now may not be as great.
I’d like to point out that Balloon-Juice was one of the few blogs that was classified as “center”.