The Wizard (link to the right) is very good at this form of discourse and a highly recommended role model for Blog Moderation.
One noticable change has occured in my reading of various sources is how often the person is attacked when they disagree with a position. I can only come to the conclusion that being polite is not a priority when online or in the news media, for that matter.
If someone makes a posting with factual errors, their conclusion is tainted at best, or totally off.
If someone states a position, I can fully see challenging them in order to see how solid that foundation could be.
If someone states an opinion, countering that opinion or asking questions is key to understanding your opinion and the other persons view.
Name calling, Labelling, Straw man arguments and other fallacy, will only muddy the actual argument and either confuse, deflect or minimize the point.
The only time to be dismissive is when it becomes apparent that the person is only interested in their own viewpoint to the exclusion of others.
It seems the English are much better at civil discource the us Americans.