And keeping one golden rule of software development in mind, don't let your system do redundant tasks. Look how many times a conversation is repeated throughout its life time in a mailing list. Thats alot of redundant data using up server traffic. In a forum everything is more central. A lot less redundancy. and you can still get topic replies. What about being able to reply to the notification email to reply to the thread? That would be a start on a compromise. Nice feature for a modern forum/mailing system. Maybe also be able to subscribe  to forum catagories. This would allow those who wish for emails to still use mail.