I've been tempted to set up a wildcat email list myself... if some one beats me to it, please, please:
Put in the user agreement that all the contributions to the list are CC-by attribution. Without that, we lose track of who really wrote it an what the license is, which super strictly speaking means it's copyright and can't be republished by anyone except the original author.
Make the archives public & accessible by google crawer and archive.org (i.e. that the service doesn't have a robot.txt file telling the spiders to go away). For example of how *not* to do it, see the kelen mailing list.
Make sure it either has a human moderator or at least a very good capcha system. Ideally appoint multiple moderators, on average people show up and participate in the tp community for maybe 3-6 months and that includes the inventor and the moderators of some of the more successful tp sites (toki lili, this forum, etc). For a good example how not to do it, see the toki lili, which has completely broken spammer detection. Many of the less active yahoo mailing lists for conlangs are 100% spam now.
And ideally, host with a company that is unlikely to go out of business for a really long time-- google, microsoft, apple. For an example of the wrong way, see all the Geocities toki pona pages. Well, try to see them, because they are scattered across a few incomplete archives.