It might not have any 3D styling, but it does critically provide a very high level of contrast between it and its surrounding page area, making it absurdly simply to find and identify as something that can probably be clicked on, hovered over or otherwise interacted with in some way.
Let's not get too hung up over "flat" here - whilst the move to the more minimalistic flat styling was a bit of a shake-up after decades of 3D-styled UIs, at least those early flat UIs still provided clear delineation between their elements. The problem is *now* that UI design has progressed even further down the road of minimalisation, stripping away pretty much anything that lets you know which part of the UI does what. In some cases the designers didn't even stop there, and continued on to remove *every* visual hint as to where the active parts of the UI were in the expanse of seemingly anonymous whitespace they so graciously decided to dump in front of our eyeballs.
So I think when people are, quite rightly IMO, complaining about "flat" UI design today, many of us are really complaining about minimalistic UI design which is still promoted by some as "flat" design, if only because those few bits of UI styling which do still exist *are* just as flat as they were in the early days of "flat" UI design...
Biting the hand that feeds IT
