This is really interesting! I use bbPress throughout a site (I couldn't figure out BuddyPress from a user standpoint) and it works very well. When I set it up, I considered Discourse but that was a year and a half ago, although the reasons haven't changed - mainly integration into the site, ease of customising it.

I am slightly surprised that no one's really mentioned that losing WP user management is important to them. It is very important to me. The users have access to a tracker of things and we might introduce paid content, functionality etc etc. Having one system to handle sitewide logins is invaluable -- it's not only easier and less frustrating for users but easier to develop new stuff (or buy it, hey there membership plugins!)

As I said, I don't understand how people use BuddyPress - I agree with Erlend on this:
"I’ve always felt that BuddyPress would be better off 'sold as individual parts'"

and very much with this from his blog http://blog.erlend.sh/when-is-bbpress-the-right-choice/:
"That said, in this case you should strongly reconsider whether it's worth having a forum at all, because in my honest opinion a forum is an "all or nothing" sort of deal. "

I could go on quite a bit. My feeling is that Discourse has a lot of excellent things that bbPress doesn't have *yet*. Discourse is great and I can see how it's the perfect solution for some sites. But I love bbPress, although sometimes I feel like the only one anywhere who does.