Can't link to another topic that has been previously discussed. Can't change message after posting. Mailing lists Convenient. Possible to work offline. Rich HTML messages are rare.
Subscription based, so people come and go depending on their interest.
Setting up and maintaining mailing lists using Majordomo or similar systems can be complicated and are nonintuitive. Content is not searchable unless archived. Threaded discussions are underused. Can't link to another topic that has been previously discussed. Can't change message after posting.