Shared augmented reality is the killer feature that I'm still waiting for. I'm really surprised that Apple didn't have that from the start. We could have things like going through your photo collection together. Or collaborating on a virtual sculpture. Or discussing ideas in front of a virtual blackboard. There are a lot of really cool things you could do with shared augmented reality.
Maybe Apple scheduled this feature for a later release? This is something that can probably be done efficiently 100% in software.
Shared photos is likely very doable, as it's just photo id, physical location, and scale/orientation. Place a picture above the fireplace and both headsets could render it exactly the same and see it.
Video would work well, audio might be a little harder to sync exactly correctly.
It shouldn't be much more complicated than what we currently have with first-person 3D shooters. All devices in an augmented room would have some existing shared data that doesn't need to be transferred. It's only the updates that need to be sent over the network.
Maybe Apple scheduled this feature for a later release? This is something that can probably be done efficiently 100% in software.