Oh my gosh, everyone is going to be talking about the head phones and 360 tracking. However, for me, the real prize is going to be the built in audio algorithms. If you're familiar with the terms binaural and ASMR, you know exactly what I'm talking about.
The most interesting VR-for-mobiles application I can think of would be the implementation of a virtual keyboard, allowing you to type anywhere. That would be a real game-changer for me. Obviously it would require motion tracking as well, though.
Integrated audio seems like a bit of a bizarre choice. I'd bet anyone with the cash and interest to buy an Oculus probably already has a decent quality audio setup, and these don't exactly look high tech. Then again, probably a moot since they seem removable for any picky customers. (Talking about the hardware side anyway; proper surround algorithms are always welcome.)