"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"
To me, cooperative, exploratory statements are the most valuable.
"That is a interesting opinion, I've never heard that perspective. But what about case xyz, have you considered that."
In that context, it is much easier to build up interesting information trees, rather than simply staying stuck on one (often minor or irrelevant) node.
(DH3 ftw!)
Edit: Irony.
It was just that when I checked back in on this about half an hour later it had already been downmodded, and that just seemed really ironic to me. Glad someone is out there stomping DH3... I would be.