I don't see any way to create rules for Discussion spaces (and inherited rules don't work). How would you go about implementing sending emails for new topics, posts, etc?
I'm pretty sure you could programmatically create a rule and attach it to the space representing the topic or forum. It's technically possible just not exposed in the UI right now.
I'm not aware of any plans to add this feature but sounds like a good idea so feel free to raise it in JIRA, you can then track any progress made.