One of the most negative habits is in my opinion the failure to read a comment charitably (to make an effort to interpret it in the best possible light). Instead, people often tend to misread or miserunderstand what's being said, only to use the opportunity to write a "correction" based on that false impression. A worse flavor of the same problem is a misunderstanding that is then used to justify outrage or personal…
This goes both ways. If I think someone has misinterpreted me, I usually apologize for being unclear and clarify my statement. I would rather clear it up than start a flamewar, even if someone else might be itching for one.