Beyond moderation

How do you have a conversation, a real honest and open conversation with someone who disagrees with you at your blog? I have found that it can be a great challenge. The issue I see coming up at the blogs I frequent of brethren who I esteem highly and who I openly associate with is trying to find the line between conversational defense of a position and advertising promotion of that position.

I guess I'm evaluating this more and more. I want people who disagree with me to WANT to post here. But I also don't want this blog to be a place where error can freely be found. I doubt I will ever allow guests to link to content that could lead some astray. But, but, but.... I want people to be able to argue their point. Where is the line?