Very good point about the "fog of war". If HN had a reply-notification feature, it would probably look differently. Every now and then someone builds a notification feature as an external service. I wonder if you can measure change in the behavior of people before and after they've started using it?

Of course, that also soft-forces everyone to move on. Once a thread is a day or two old, you can still reply, but the person you've replied to will probably not read it.

There's also the fact that there's no alerts about people replying to you or commenting on your posts. You have to explicitly go into your profile, click comments, and then you can see if anyone has said anything to you.

This drastically increases time between messages on a topic, lets people cool off, and lets a topic naturally die down.