I think we all know the issue - you comment on someone's answer but won't get notifications when you get a response to it. That makes things unnecessarily complicated in my opinion. All you can do is keep returning to the thread and check if there is a new reply. Again and again. Or follow the question and keep checking the live feed.
Another annoying thing this creates is that users will (understandably) post their replies to the other person's answer and vice versa, so that they both receive notifications. In the end this means answers with whole monologues under them. And that can get pretty confusing.
Anyways, I think a good solution to this would be to let users choose answers they'd like to receive notifications on, whenever it gets a new comment. Preferably this would be a checkbox you can uncheck anytime, when you don't want to receive any more notifications to that answer anymore.
i support this. and also an optional notification for question updates too. you use to get those in old gag but got rid of those. sometimes the QA has new information to add and would like anybody who wishes to address them to add their thoughts too.