What he said (Larry H). I type fast but write slow - edit, re-read, edit again because it sounds stupid, so I don't comment often. Sometimes I'll spend 45 minutes on a comment and just delete it because I don't like the way it reads or I think it comes across wrong. Needless to say, I don't have a blog!
I comment when a topic resonates with me and when I feel that I have something to add to the conversation. Otherwise, I'd rather not rack my brain to come up with something that sounds spontaneous and sincere, when it actually isn't. I subscribe to many blogs and only have time to comment on a few. If I'm going to leave a comment, it has to be more than just two words. "Great post!" is a cop out. If I can't do more than that, I don't comment at all.