[eyevariety]

Its not that a simpler shape isn't easier to process, its that its harder to infer purpose when there is so little information (like in the case of Metro). A button is something that needs to be 'pressed' so it should look like it can be- and by doing so you understand that it is a button much faster.

Donald Norman and Jakob Neilsen have long preached this and they have a bunch of info on affordances and why they work: http://www.jnd.org/dn.mss/affordances_and.html

More recently see Steve Krug's "Don't Make Me Think"

[zevyoura]

Interesting, though I have to wonder if that's necessarily true; it seems that you could designate a button using only rectangles and solid colors in a way that would be clear and unambiguous (e.g. Google+'s buttons). Either way, interesting application of Norman's ideas of 'affordances.' DoET is a perennial favorite of mine.

[panacea]

>(e.g. Google+'s buttons)

They actually have slightly rounded corners, an outline and subtle gradient going from top to bottom that hint at three dimensionality and offers affordance. There's also a slight darkening on mouse-over which suggests they're interactive and can be pressed in.