It needs an App Store. That's all. I'm tempted to buy a current model now with the hope that a firmware update (is such a thing possible?) enables the Store.
don't do it!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Seriously, current model is A5 with 8GB. While it will get iOS 9 and likely Apple Music sooner than later, I really doubt it will get AppStore and a lot of the new features of the new box.
New box has been leaked to have an A8 and 16GB, on top of Siri and a touch remote. It is basically like going from an iPhone 4s to an iPhone 6.