[ssdspoimdsjvv]

Online discussions lack tone of voice, intonation, etc. and any form of non-verbal communication which makes it really hard to judge the intention of the other person.

And of course pure anonymity makes it possible to be an asshole without having to suffer the consequences.

[robbedpeter]

Consider the absolute freedom of 4chan folks. They can say literally anything, and land in good discussions whenever they choose, if they're willing to tolerate the noise. Can't really get that in real life except among really close friends.

[Ntrails]

Basically there are not many places you can take a premise at face value and just throw thoughts out without worrying about consequence to social standing etc.

[afarrell]

They also lack the ability to signal listening.