Nice to know that I got some of this 'homework' done early, by commenting on various blogs at earlier times in the course.
For instance, when I found the languagehat.com blog during RSS feeds searching, I read several fascinating posts about word origins (one of my personal passions) and was particularly struck by the discussion on "gabions," those wire mesh pillars filled with rocks that we see on construction sites (and as corner fenceposts in less-wooded desert country), as the comments led into the related "fascines" which are bundles of pipes/sticks which can quickly be dumped into a ditch to provide a stable 'road' for tracked or wheeled vehicles, then taken up again and carried until needed again. That reminded me of the word "fasces" as in the bundle of sticks bound around an axe, carried by Roman lictors as sign of office, sooo.... I commented with that observation. Made me feel so darn smart, finally using some college research work after all these decades.
I have also commented on Library2Play blogs when I can specifically know that they are also school librarians - sort of that common bond thing.
My pet peeve with commenters is the "This is Photoshopped" as first comment game - yeah, I know that they don't really think it's PSd, but it's still annoying to have to slog through those sorts of comments. Then the "me, too" or "really cool/cute" oneliners are pretty much space-wasters, as well. Unless the blogger is asking for a vote/consensus-by-comment, I won't post comments of the "great post" nature.
Some commenters on Drape's Takes Edublogger Etiquette series http://drapestakes.blogspot.com/2008/05/edublogger-etiquette-responding-to.html felt that they could not go back to comment on a post after taking the time to digest it and think it over, feeling that the blogger and readers had "moved on," but I think that it's okay to revisit a post with comments if you have something authentic to add to the conversation, whether or not it's within a quickie time window.
I'm not so sure about the continued interjection of the blogger into the comments-conversation when it's just "thanks for the comment" type comment. To me, that interferes with the flow of the conversation, unless the blogger poses further question for all =- but then, that would more rightly call for a new blog entry, with reference to the starter entry.