When someone leaves a comment on my post, I get a notification mail. The comments can sometimes and most times lead to some discussions.
The comment poster is never notified if someone has replied to the conversation thread he started. He should volunterily keep track of his post for the conversation. Thats a bad thing.
Having a selection to get notified when someone posts a reply on the comment will be a great feature.
LiveJournal has this good feature already.
I had this in my mind for a long time and today I just suggested this to the blogger folks