The other thing that is not entirely obvious is that Playgrounds has full access to the entire iOS API. This means that there is effectively no limit on the complexity of Playground that you can build. You can use APIs like Core Location, WebKit, MapKit, Core Motion, Networking and Core Bluetooth. One of the demos given in the session on Playgrounds was of Playground Swift code controlling a Sphero robot over Bluetooth. I already did the standard daft Browser-in-five-lines-of-code trick that we used to do on the Mac.
I'm looking forward to playing with this app. The demo looked great.