Mailing lists are good for some things, and really really bad for most. In particular they scale poorly, both up and down. Many conversations suffer from crowded public space which a mailing list inhabits, one in which a rapid back and forth is either discouraged, or if allowed brings all other conversations to a screeching halt. One solution many people try is, “Start a new list”, which is why lists.indymedia.org had over 600 mailing lists last time I checked. Another is the private CC list which has the problems of being transient, difficult to join, difficult to remove yourself from, unstructured and unarchived.
I’ve proposed using quicktopic.com in the past, but people don’t seem to want to leave their email readers, and kicked around the idea of using Roundup, or at least extracting the ad-hoc list code from it.