Aren't most DEs not solvable analytically? i.e. only a tiny subset are neatly solvable - like precisely manufactured puzzles. In general, for any real problem you encounter "in the wild", numerical methods are the only way to find solutions (which are approximate).
DE theory is still useful for designing frameworks within which numerical methods can operate.
Yeah, I think "mettle" here is like a sword and shield and the ability to use them (read and understand scientific literature, and able to do so, self-directed, for years).
But whether a person does use them, and for what, is entirely due to the person themselves.
Shadows help ground images (e.g. characters in games seem to float without contact shadows). Seems simple, but requires a 3D model of the room to work properly. LiDAR
Lag between reality and virtual would be noticable side-by-side, no matter how much the "R1" chip reduces lag (12ms)... Ah! they might do the Guitar Hero trick, and delay the reality feed by the same amount, so they are exactly syncronized... see-through AR (like google glasses) can't delay reality.
The outward display of reverse faux-transparency is a nice touch, displaying a fuzzy image of the wearer's eye on the front. (Though not sure what to make of the back-side of the displayed image - could a user ever see it?)
I think this is the first major Apple developmemt without Jobs' involvement?