I think it would be a much better idea to have StackOverflow-esque on-site notifications, instead of constant emails. Users could still subscribe if they like, but wouldn't need to as much if we have this.
First, there would be a page in their user profile which shows all replies to their posts - answers to their questions and comments on their questions/answers.
Second, when a user returns to the site, a simple banner or icon showing that they have new replies since their last visit. The link would take them to their replies page.