Here’s some pet peeves of mine when it comes to commenting:
Blog advertising – comments of an advertisement-like type will be deleted – don’t use a comment on a post made here to advertise your e-book or anything else. Such comments will not be approved.
Hurt feelings when your assertion is challenged, or rejected. Believe it or not, a random comment on my blog will probably not dissuade me from what I believe.
Hurt feelings when something you believe is called out as shallow, dumb, goofy, etc. If I think that what you are saying is dumb, I will say so. No, this isn’t a personal attack.
Comments which don’t really engage with the topic but use it as a launchpad to present the commentators views as the right view.
I don’t mind disagreement, or vigorous disagreement. I don’t even really mind if comments aren’t friendly. But I do mind when the content is shallow and the arguments are bad.