This is like moving from a small town to a big city. Someone can be rude to another person in a big city because more than likely they will never meet a again. In a small town everyone knows everyone else and word spreads rapidly. You don't want to act like an ass in a small town because everyone will hear about it.
In large online communities, karma is supposed to act as an incentive to be nice and contribute thoughtfully, but it is not as effective as true reputation among people that you know personally. What the world as a whole thinks of you is less powerful than what your friends think of you.
A good experiment would be to enforce the small town effect on an online community. Partition a news site into groups of 100-1000 members. Comments and submissions would only be visible inside each members subgroup until upvoted past a certain threshold. With luck members would get to know one another inside a group and desire each others respect enough to contribute to the discussion meaningfully. Debates would last for days or weeks instead of an afternoon as is the case on current news sites. Trolls damage would be confined to one group at time as well.