Replying to my own message, as I thought of additional goals:
- Users want replies to go to "the right place" without thinking about
it. "The right place" depends on the content of the reply and cannot
be determined reliably by looking only at the message being replied to.
- Recipients don't want to receive duplicate copies of messages, even if
they arrived over multiple paths. Senders don't want to have to remember
who is on which mailing lists.
- Users want a uniform interface to mailing lists.