Jon Udell writes about E-mail's special power and points out, that a lot of the usefulness of email comes from its utter simpleness. A particular strong area is instant, low overhead group formation, which often renders email the preferred tool over more powerful but also more complicated tools like web forums, KM tools, etc.
Clay Shirky then points to an important missing feature in this instant group forming: The possibility to easily get unsubscribed from a discussion. In e-mail the sender determines the group, and the receiver has no easy means of leaving the group.
On the other hand I often find myself reading a partial thread, because some people answer in private or take me off the list, while other's add me back. In these cases I'd prefer a few superfluous mails to having to reconstruct a discussion by combing quoted parts from different mails.
Ross Mayfield adds, that they allow CC to wikis as a means to complement the group forming use of email.
We use the roundup issue tracker, which has a very interesting feature called nosy lists. You can discuss an open issue in email by CC-ing roundup (which tracks the issue with the subject line) and all users that are CC-ed are automatically subscribed to information about this issue. The important point is that issues are generally short-lived, so that landing on such a list doesn't mean too much irrelevant mail in turn for being sure not to miss important information. Another use I like about it is the automatic archiving of the discussion.
The interface could be improved further: First, tracking with the subject line is brittle. The first mail (which opens the issue) doesn't even have to correct line and any Reply-All answers to this mail opens new issues. This could be solved by relying on the "Reference" headers in the mails instead. Secondly, unsubscribing is too much work, it'd be nice to add a link to outgoing emails that points to the unsubscribing page for this instant-list (keep in mind however, that a mail could come directly from a sender who uses reply-all and thus, the link cannot incorporate the user information; just the list information).
This would be a nice standalone application or a nice addition to a wiki software, too: Some archiving address in the organization that manages short-lived mailing lists and publishes all mails it receives on the web, threaded and searchable.