Here's a perfect example of how comment systems don't "scale" (in the social sense) - any site that covers controversial topics (especially political ones) will end up getting overwhelmed by commenters picking fights. Here's what the Washington Post had to do:

At its inception, the purpose of this blog was to open a dialogue about this site, the events of the day, the journalism of The Washington Post Company and other related issues.

...

But there are things that we said we would not allow, including personal attacks, the use of profanity and hate speech. Because a significant number of folks who have posted in this blog have refused to follow any of those relatively simple rules, we've decided not to allow comments for the time being. It's a shame that it's come to this. Transparency and reasoned debate are crucial parts of the Web culture, and it's a disappointment to us that we have not been able to maintain a civil conversation, especially about issues that people feel strongly (and differently) about.

This happens on any highly trafficked system - popular usenet groups became useless piles of dreck years ago, and blogs and forum sites are merely following that trend. Look at slashdot or digg, for instance - for every useful comment in a thread, there are dozens (sometimes hundreds) of comments that - boiled down - say "you're a moron" (only in less polite terms).

Any site that gets popular is going to end up going where a lot of the popular political blogs went a long while back - comments off. The only trouble with that is that tracking referers is hard, due to the absolutely enormous volume of referral spam.