It's not just the fact that "people can recognise a button even if it's flat" that is the issue (I often hear people making this argument) but that the differen

It's not just the fact that "people can recognise a button even if it's flat" that is the issue (I often hear people making this argument) but that the differentiation between what is a button and what isn't (e.g. what is normal text) is reduced in flat design. It is possible to make 'non-buttons' work as buttons, but only if their function is clearly indicated via other means. This is the beauty of the traditional, three-dimensional, consistently-styled buttons: you get a 'button' by default without having to hint that it's a button using other means.