While I suppose it does accomplish the purpose of decreasing heat in threads, for me it's a big usability issue since what usually happens is, I would comment on something, then forget about it and only check back days later, at which point there are several relies to my comment. Then, at that point, if I do reply again, chances are the other person won't reply back because he has no knowledge of my reply.

I think this is one of the best features of HN. It reduces a lot of the energy around receiving replies to a comment on all sides, there's never this neon light going off "hey, you need to respond to this right now".