I think most people are really nice and polite on here. Maybe I spent too much of my youth on Stormfront. I just consider the fact that I don't actually know anyone here. I don't know what they're dealing with in real life, and I don't know why they're here or what they're looking for. After my initial frustration with something someone says, I usually consider the fact that it's difficult to tell what unfamiliar people are saying, or how they actually meant a comment, when you're online. This is especially true if they don't use lots of emoticons.
There are times when I don't understand my own close friends when we're online due to a lack of body language cues that help you understand another person's intent irl. It's occasionally difficult to know when someone is teasing, or being playfully sarcastic, or actually meaning to be hurtful.