add syntax for mentioning another user, perhaps @username. when a post containing mentions is posted, that user is notified about that post.
perhaps also support mentioning groups of users, eg all the moderators of a given forum or all the site admins, or something
each user has a set of threads they are 'subscribed' to. there is a button to subscribe/unsubscribe on every thread. when you create a thread, you are by default subscribed to that thread.
there is a 'notifications' page that lets you view unread notifications.
presumably there is some system to work out which notifications you have read and haven't read. i will think through that later.
also, if we separately have a 'number of unread posts' field on each thread which is separate from this notifications system, then that will use the same system as this, presumably. (presumably it will be useful to see the number of unread posts on threads you aren't subscribed to)
once we have subforums, you will be able to
subscribe to a forum, and be notified when new threads are created there
(separately) enable auto-subscription to all threads created there, and be notified of all posts created in those threads
subscribe/unsubscribe to all existing threads at once easily.
presumably we should avoid notifying a user twice about the same post (ie via subscribed thread and via mention)
but if a post is edited so that it now contains a mention we would want to notify them
if a post is deleted presumably notifications pertaining to that post should be removed. if a post is edited so it no longer contains a mention then the notification of that mention should presumably be removed. (we should think about what ux makes most sense here)