I loved the nested discussion of mailgroups. Compared to message boards and forums of today (yesterday? I think they may be passe?) they enabled interesting, meandering discussions. It's the way discussion flows between two intelligent people. Forcing discussion into an arbitrary topic has 2 problems:

* some people are unhappy because they feel constrained * topics are an illusion anyway, people do post off-topic messages anyway, depending on strictness of moderation. People interested by the original topic are annoyed by the inevitable off-topic talk.