It has nothing to do with anonymity. It has everything to do with the content on which people comment. For example, political articles are bound to bring in opposing viewpoints. And if you allow anyone to comment, you're bound to bring in a lot of people.

I'm convinced if you want to keep the level of discourse up, you can have a (mostly) homogeneous group of commenters and a large scale site or a small group of heterogeneous commenters, but you cannot have both scale and heterogeneity.

By charging your users to comment, you're probably going to reduce scale.