I would love it if I could get an e-mail when a story's accepted. The rejection e-mail is helpful, but it'd rock if Tracker would ping me when someone's signed off on a story after I've finished it.
And as to pie-in-the-sky ideas, I know a bunch of people here would love it if there were an "e-mail me when this story is accepted" button on every story, so you could have Tracker update you on stories that aren't your own.
When I create and assign a story to someone else, it would be great if Tracker would send that person an email letting them know that a new story has been assigned to them.