Put simply, when you are wearing a camera on your head, all mirrors you look at also become cameras.
The consequence of this is a simple pocket mirror can be used as a communication device, allowing skype-like communications anywhere.
Because mirrors reflect the full lightfield, any HMD using optical based systems for 3d measurements can also exploit the mirror to record you in full 3d. This opens up possibilities like replacing a background by using a simple depth test to create a mask.
Note: The mirror itself will appear, in terms of depth, to be a discontinuity in the environment; a hole where there should not be one. For HMDs with some sort of sense of depth, this should make them relatively easy to track.
In short, if AR is a way to have everything everywhere, then mirrors become a cheap, simple, way to talk to anyone anywhere…..while also looking like anything you’d like.