It tends to isolate people into self-reinforcing communities, many of which have two things in common: They have an enemy that they demonize, and they admit no wrong.

Not all communities are like that, but when communities (and now that I think about it, especially ones where your comments can get some kind of point or reward) get big they trend towards that. Possibly because those behaviors are low-hanging fruit in terms of your comment earning points. Points feel good. No one likes to admit it but they do.