There would be two ways to do it that I can think of. Either there would be an RSS feed for each thread, and you would subscribe to all the ones you wanted to monitor with an feed-reader, or you could have everyone get their own RSS feed for all their subscribed threads. The first one makes a lot more sense to me.
My inbox gets clogged with notifications all the time and it gets kinda annoying. This is what feeds are for!
