Object occlusion like they do with the floating robot is also really hard. It's easy to kind-of do, but hard to do that crisply.
A subtle problem is where to put things that don't just add noise and clutter.
Look around your house or office, where would you like digital things to sit? Ok how about when you move 2 feet this way, whats the new answer for where you would like it so it's easy to see/interact with?