iOS App designer/developer Jared Sinclair:

What makes something touchable?

For things that scroll or zoom, touchability means that the content under your finger moves with your touch, without any lag or jitters…

…For buttons, touchability requires something different. Touchable buttons need borders. By “borders” I don’t mean outlines, (although outlines are included in my usage of the word). I mean borders in a broader sense. A button is a tappable area, clearly delineated from the un-tappable content around it by an obvious border.

Native app design isn’t my background, but the switch in iOS 7 from clearly defined buttons with borders and gradients to raw text labels always rubbed me the wrong way. Jared makes a strong argument why. (via Jeffrey Zeldman).