I want members to be able to communicate about various ideas sometimes without direct organizer involvement. A message board would allow this, and the ability to organize conversations by area, or by topic. I don't see this available... if it isn't, I'll upgrade this to the suggestions box...
Currently the best page-type to use for this is the suggestion box. Here is an example of how we use it as a message board/forum on our own site: http://nationbuilder.com/website_questions
If you think you might want to have greater division of sections, you could create a basic HTML page with a list of your categories and link each of those to its specific suggestion box.
We do have a forum page-type in our development queue. If you think this would be a valuable feature to have, please add your comment to a current feature request here: http://nationbuilder.com/103136/put_up_a_real_forum. If it gets enough traction our engineers may push it up the list.