Derek Powazek lays out a commenter's bill of rights, one which gives them much more power over their words than most current discussion systems offer. (He makes an exception for Kinja, which is nice.)
I wouldn't give commenters quite as much control and privacy as Powazek urges. He has a binary an idea of private and public communication: semi-public is often optimal, a quiet room for the incubation of conversation, open to all but not advertised widely. That's often the best environment for productive discussion.
And why should commenters be able to take back their words if they regret what they said? We don't give that power to politicians. You can apologize, but the record lives forever.
At the point at which someone responds to your comment, you no longer fully own it. You're part of the conversation. You can't just take your words back, when others have based their own words on yours. It would be like the removal of Einstein papers on relativity, the foundation on which a century of physics had been built.