1- Email notification to the author of a topic when a response is made to their thread.
2- Ability to subscribe to a news or article thread for a variable duration thus being notified of replies to an article you did not actually post yourself.
3- As admin, be able to subscribe a select number of members to a thread.

I find that PHPNuke and it's siblings are the only ones without such a feature - geeklog and others have this ability and I sorely require this as a solution to a looming problem.