You can't just declare evidence against your claim "exceptional". Reddit has a large userbase, and includes numerous communities. Nested discussion seems to work pretty well for it.

Yes I can!

The point is this: reddit has a certain demographic that you don't find on most websites: mostly tech geeks that embrace modern web technology and a very high proportion of programmers. On the other hand look at slashdot, horrible discussions, just a random pile of opinions with many flame wars.

That said, discussions in News forums (news paper, news agencies etc) are often very stupid or unpolite. But I have seen that they are at least slightly more polite in flat forums. There is less repetition and if someone complains about unpoliteness, this is taken more seriously by the followups. Of course flat forums are way more work for the commentator because he needs to read all the posts that were before and make conclusions based on them. This can be next to impossible on an article's comment section with high load.

I think reddit's moderation system has more to do with the relative success of its discussions than its audience. Are you implying Slashdot didn't have a high proportion of programmers?

Yes, at least a lower proportion than Reddit. But maybe I'm wrong.