In some cases this can cause a user to miss a reply in a discussion in which they had participated recently. Some users consider this to be a feature rather than a deficiency.
Yeah, I'm kinda glad HN doesn't have that. I think it helps avoid prolonged, drawn-out arguments that just go around in circles, generate animosity, and lead nowhere. Not getting notifications makes it easier to just let a conversation die out. I would put this in the "feature" category myself.