I understand that notifications could be helpful in some cases, but I actually like the fact that people don't get notified when someone replies to their comment, and I had assumed this was deliberate. It seems to make debates more objective. For example, if user A makes a statement, and then B disagrees, I like the fact that C can come along and backup A's points, rather than a defensive response from A. (Having notifications that were delayed by an hour or so might make for a good middle ground.)