Personally, I want clearly-labelled tactile buttons that make me want to push them. Flat UI is more brutalist than humanist.
Relying purely on scientific data to create an attractive/useable interface can take a million years. The human brain is not a complete a black box as we have the capability of understanding ourselves to a certain extent. As a result, we also have certain awareness of what interfaces we find attractive and useable. AB testing not required.
Do you really need an AB test to determine whether or not hacker news should have a background that switches between black and yellow every half a second? No. Interfaces should NOT be data driven.
Interfaces need to be design driven, and data supported.
With regards to the flat trend it's deeper than just everything suddenly being flat. What Google and Apple is trying to develop is a visual/interaction language where the content itself becomes both the content and the interface at the same time.
It's not just about how it looks its as much about how it moves, the choreography so to speak. It's as much driven by trial and error as it is by data and thats exactly how it should be.
I suppose I could investigate, but I don't need google to do anything worth investigating.
My phone has a "tiles" button, too, but it apparently means "menu." (There are multiple such buttons, each looks different.) It also has a search button I've never pressed.
I feel that iconography is far too overused, and it is especially unsuitable for the kinds of abstractions we use for software. It's one thing to put a toilet or a phone on a sign, and a completely different thing to put a circle with a wedge, or a pair of rectangles on a sign.
You are confusing my personal feelings for a rigorous test suite. Your point didn't concern how I feel.