"Which brings us to the most important principle on HN: civility. Since long before the web, the anonymity of online conversation has lured people into being much ruder than they'd dare to be in person. So the principle here is not to say anything you wouldn't say face to face. This doesn't mean you can't disagree. But disagree without calling the other person names. If you're right, your argument will be more convincing without them."
Some people actually do call names to others when face to face.
Personally, while I don't, I do tend to get a little aggressive and then I'm often surprised with the backlash, because I get that way when I'm genuinely enjoying the conversation, not when I'm irritated.