i think a great feature for mailman would be some indication of how busy a list on the web page showing all the lists. (e.g. this page: http://mail.python.org/mailman/listinfo)
e.g. 15 messages a day, or 3 messages a week, or 2 messages a month
(averaged over the past month)
this would listed as a new column between the name of the list and the one-line description.
i don't know anything about the internals of mailman, but i think it would be really useful info for people to know.