One of the biggest problems with large mailing lists is that there is usually a core group of really good contributors, and then large masses (often new subscribers) that just send out the same "How do I...?" or other "noise" type questions that could easily be answered by reading the manual or checking out the frequently asked questions page.
This article by K. Scott Allen made me think about some changes to mailing lists that might help with the signal to noise ratio.
Each subscriber could set their threshold, so some users might choose to receive only the best posts, from the core group.
New members might have a certain trial period, or some other way for their posts to be seen following their initial sign up.
Karma rankings for individual users and/or posts could be done by a set of moderators, through a web interface. Alternately, any person could give feedback about a post by sending mail commands back to the list server -- e.g. forwarding it to a separate address, like "list-karma", and having +1, -2, etc. listed on the first line.