How to raise Notifications on new discussion posts?
You can use Flag module to create "subscribe button" on any content (forum category, post or comment) and then Rules to create rule:


what reacts to new content
then load parent entity (if new post, load it's category)
load all flaggings on it
send e-mails to authors of all flaggings OR create notification (so you can show them on some dashboard)


There are also modules specialized for subscribing to content and then send notifications, but they usually are not so flexible like solution based on Rules.