An interesting concept for a forum community. I know not of any other forums that use a system like this (which is not to say they do not exist, I am hardly well traveled in forums) however I think that this would work, providing it has the aforementioned toggle ability.
Perhaps an important feature would be the ability to choose which threads you chose to follow. Not all of the threads I post in I wish to be notified of every time someone else has posted. I frequently have one thing to say and feel that, for my part, the matter is closed. Also, there should be a feature that somehow (although a suitable solution is not obvious to me of the top of my head) to very active threads. I have little desire to get constant notifications and would rather have a notification for every ten posts, or every set amount of time, to remind me to check.
For the moment I use the save post function to facilitate this but it is hardly a brilliant solution. I am interested however in what the rest of the community thinks of this and what solutions they come up with.