I'm fascinated by how much people are using the Reminders app, especially as their main GTD list manager. It's a minimalist app, but useful. I use it too, but only as an adjunct to my main system (OmniFocus and BusyCal, which I like better than iCal). It gives me an easy & reliable way to jog my memory for little things like stopping at the store on the way home.

I believe I've read that iOS 6 (which is presumably due out next month) will add sorting capabilities. I'm not sure if any other features will be added.