My blog, like most blogs, allows me to express an opinion, share news, write reviews, or whatever, and then people can comment on them. If I choose to, I can reply, but the comments are really secondary... normally. (I'm not trying to say they are unimportant, but you get the idea.)
I belong to a very small listserv with a few good friends. These are people who - while I seem to disagree with most of them - I have an enormous amount of respect for. We are currently discussing recent decisions by the FCC and basic ideas of what our country should allow corporations to do. I think it's a great discussion (although my friends are completely wrong ;). I'd love to share it on the blog - however - the comments system isn't really well suited for something like this. We could make one very long blog entry - and use different colours and stuff to represent each author - but that doesn't seem right either. Comments - as I said - wouldn't work. I'd want this discussion to be limited - not because I don't value your input, but because I'd want to keep things focused. It's kind of like - how would a blog handle a political debate... which would have a few participants, but many viewers? A "member-only" theaded discussion could handle it - but how do you integrate that into a blog setting.