3/ If waving your hands in front of a real giant screen didn’t work before (I don’t believe we’ll see a touch screen iMac), it’s hard to expect that waving hands in front of a virtual screen will. Especially for high-precision tasks w/o haptic feedback.
4/ If productivity AR apps (and maybe any AR apps in general that need complex input) ever take off, I think we’ll see the next wave of skeuomorphism. You’d manipulate something close to real objects on a table or in your room instead of flat controls on a floating flat screen.