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.