Grendel, might have a clearer view than me.
It's tough. Some people are looking for engaging polar viewpoints without vitriol, and it happens, but IMO it's almost as destructive to force that to happen over the entire site as it is to make a 'safe place'.
So, what we do, is put moderation in the hands of individual posters. At best, what happens is that people that don't want such discussions don't host them, but those that do can. I think it works better than a top down approach, but it's not perfect in this regard. I am not sure if there is a perfect solution.