Back in the days of dial-up BBS systems and then Usenet - threaded mechanisms were very common, and this feature is, I believe, one of the greatest factors in their usability. The old forums on Compuserve were also a good example.
However the move of many groups to plain mailing lists makes it far more difficult to follow threads of conversation. Several groups I know of have gone to Yahoo, for example, and it is now far more difficult to manage. Excellent threads get lost in the dross of the daya to day, usually irrelevant, noise that these lists generate when un-moderated. Sadly moderation doesn't really work either. Self moderation by thread management is far more effective IMHO.