My comments on other blogs were a bit forced - to complete this assignment. I guess if you frequent certain posts and get to know the bloggers and regular commentors, you might find more value in contributing.
One blog I really liked was blog.mrmeyer.com which is a math teacher's blog. Interesting posts and interesting comments. But some of the comments were really long (longer than the original post). Where do people find the time? Also seems to be the equivalent of "like hearing the sound of your own voice."