Maturity & confidence is a big part of it, DS(13) soon to be 14, is mature for his age, 6'1". We grant him quite a bit of freedom, especially during the day time. He has a cell phone which helps all of us keep in touch. In the morning, he'll go down to the Boardwalk bakery to get the morning croissants or at night he'll go down to the arcade no matter which resort we're staying at.
He's not been to the parks alone, that's not much fun really and he's never asked. We're lucky that we've always been able to bring 1 or 2 of his best buds with us, and as long as it's 2 or 3 of them together I let them go without adults. Between the 2 or 3 of them, they have enough sense to do the right thing and to keep the lines of communication open, so that we can catch up to each other without worry! We've left them at Disney Quest for 1/2 a day while we caught a movie and met up afterwards for lunch or dinner!
On the few times we've gone without friends,I've let him run around BCT/BWV without adult supervision, and if he wanted to stay for the movies, I'd be ok with it. He's been to the resorts so many times, he knows his way around and considers them a home away from home. Someone new to the experience might be intimidated.