iOS 7’s designers have abandoned bordered buttons in favor of borderless colored text. I think this choice is unjustifiable. It is the root cause of my deep dislike for how it feels to use iOS 7. It introduces unnecessary tension and makes everything less usable than it ought to be.
Color alone simply cannot be the way to identify a button. You don’t touch a color. You touch an area. To activate a button, you must touch a spot inside of its boundary. Text floating in the middle of vast whitespace doesn’t define a boundary. Only borders define boundaries.
Cf. Jakob Nielsen’s finding that flat design hides calls to action.