This is very much my experience too. There is also a very high variance in quality of discourse in face-to-face situations.

I think it's slightly easier to have moderate-to-high quality discussions in asynchronous online writing (assuming that's what the participants want), because you can treat stuff-you-can-Google-easily as an assumed baseline of knowledge and competence.

A silly idea I have is to model the quality of conversation as a random walk. With no boundary, you will almost-surely sink below the YouTube Comment Event Horizon as time passes. But if you have Wikipedia as a lower bound, the average quality of discussions will tend to increase over time.