In the process of helping my Pacifica Graduate Institute classmates start an email discussion list, I collected some what I've learned over the past decade as a participant in and moderator of several such lists and thought those insights would be worth sharing with a wider audience.
Each list should have a "mission statement" that explains the list's purpose and, by extension, who the potential list members are and what discussion topics are welcome. It doesn't need to be anything elaborate, and it will likely change over time. Such a statement serves two purposes: It provides is a standard against which to measure whether the list is doing well as a whole. It also gives list members a benchmark to help them decide whether a message should be posted to the whole list or sent to an individual.
Many discussion lists have rules that are enforced, sometimes arbitrarily, by a mysterious unseen entity ("the moderator"). If the list's rules and moderation policy are something beyond common sense standards like no spam, no profanity, and no off-topic posts, they should be posted somewhere so that all list members can refer to them easily.
Almost no one reads every word of every message. The chances of your message being read all the way through are inverse proportional to its length. Also, you should not assume that someone understands the context if you refer to some other message, unless you quote the text that you're referring to. Furthermore, if your message requests some response or other action, you might want to put that in the first paragraph, because fewer people will see your request if it appears farther down.
From time to time, someone will send a reply to the list that contains personal material that probably should have gone directly to the original sender. It's embarrassing, but it happens. If you're taking someone to task over their opinion on an issue, for example, think about whether you want that conflict to play out where everyone on the list can watch or whether you want to make it a private conversation. Remember that, when you reply, you can change the To address to reply to an individual, rather than to the entire list.
It can take quite a while for a discussion list to find its collective voice. It's less a matter of making rules than experimenting to see what "fits" – what kinds of messages develop into the kinds of discussions that list members enjoy enough to engage in. The one list in which I've participated for 10 years has been successful, I think, because it found the right balance of subject-matter content (questions, comments, and answers on the topic around which the list was organized) and personal content. We've supported each other through job losses, house moves, divorce, death, serious illness, children with problems, and children getting married – and at the same time, we've ranted and raged with great passion about technical topics.
Finally, Richard Tarnas, author of The Passion of the Western Mind and Cosmos and Psyche, has some very useful things to say about email that, I think, apply particularly to discussion lists. He urges anyone who is serious about their writing to make every email message count: Proofread it, and examine it for feeling tone and possible misinterpretation. That discipline, he says, will make your "important" writing (what you write besides email) come that much easier. In my opinion, such care also shows how much you honor the recipients. In the context of discussion lists, I would add one other thing – consider how your message contributes to the purpose of the list.