This is another cool feature. Communities now have "discussions". Discussions are like simple forums based around a topic. Any member of a community can start a discussion, and other members can join in. It has full threaded reply capabilities, and you will get email notifications when someone replies to one of your comments. You can also apply visibility levels to a discussion - you can either make it public, or just for your community to see. A community can have as many discussions as it wishes.
The active discussions are listed on the community home page, in the main column underneath stories.