Hmm, maybe there's a bit of confusion on my part.
What I'd like to see is a smarter subscription mechanism. By smarter, I mean that you get notified once when a watched thread has a new reply, but you won't get notified again until you visit that thread.
Of course, I'd like to see this as an option. Some users like to following a thread purely through email, while others prefer following it in the forum. An email per post gets a bit unruly in really busy threads for the latter group.
Also, I'm not sure this needs a cron job to manage?