[lucas9000]

i think it's a combination of two things:

1) the lack of accountability on the internet. as you point out, a lot of what people say to one another online would not be said face-to-face, because the interweb generally lacks the accountability that personal interaction facilitates.

2) the difficulty in translating sarcasm/teasing to written text. in person, somebody might say something that if taken literally would offend, but said in person would convey sarcasm or harmless teasing. the difficulty in conveying those things in text online leads to misinterpretations.