In addition, a mechanism which notifies people when they haven't had the last word probably would be detrimental to the overall tenor of HN.
Edit: I see nickbarnwell beat me to it, but I'll upvote his reply and let mine stand anyway.
Emails are intrusive in my eyes and clutter my inbox, I just want to be told when someone has replied when I visit the site.
You'll never get a reply if you respond to something someone said yesterday.
Additionally, it sucks having to click two times (User > Comments) just to see if someone's responded every few minutes/hours. I've got better things to do. Notifications are a passive feature; you can choose to ignore them or turn them off, but I think they are vital to sites that tout their userbase and the intelligence therein. You could almost say Reddit's success can be hinged on the orangereds.