I found this article on Digg:

The web is turning writing into a conversation. Twenty years ago, writers wrote and readers read. The web lets readers respond, and increasingly they do—in comment threads, on forums, and in their own blog posts.

Many who respond to something disagree with it. That's to be expected. Agreeing tends to motivate people less than disagreeing. And when you agree there's less to say. You could expand on something the author said, but he has probably already explored the most interesting implications. When you disagree you're entering territory he may not have explored.
How to Disagree.

We all know we are guilty of using DH0-5 sometimes, instead of DH6. I think we should use these labels in discussions. For example, if someone responds "YUR A NAZI RACIST", instead of replying something worse, you just reply DH0, and everyone knows not to take that argument seriously.