[frooyo]

To motivate user interaction (sign up), the best approach is to do something like this:

http://www.streeteasy.com/nyc/talk

Where for non-logged in users, you display the comment field to post a reply. But once you click submit, it asks you to login/register.

I talked about this in a previous post –>

https://bbpress.org/forums/topic/allow-anyone-to-post-to-bbpress-including-non-logged-in-visitors