I receive a Notification when someone comments downthread of me

(User Story, To be Fixed , Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Notifications
Summary: That is, the default should probably be that commenting in a thread "subscribes" you to that thread.
This wouldn't be hard to implement -- indeed, most of the work would be adding the "mute" feature to turn it off. How crucial is that?