I dislike when differing points of view arise in a thread and people try to get mods to close the thread b/c they're offended. I also hate when they try to get everyone to "agree to disagree" so they don't have to restrain themselves from reading things they disagree with and continuing to comment.

Additionally, I don't understand when people take things really, really personally on an Internet message board and accuse others of having nefarious stereotyped views about X,Y, or Z.