When replying to a post, "Email Notification: emails sent to you whenever someone replies. Only registered users are eligible." does not do what it says it does, it actually adds the thread to the Subscriptions section of the user control panel.
I think this is off by default now, but if anyone is using this feature, the Subscriptions tables in the database grow and grow, taking up valuable CPU time every time a person responds. Best course of action is to eliminate the feature altogether.