It seems to me that visual clues can be used to show people what the
button will do. For instance, the case of play/pause, perhaps it's
enough that when the mouse runs over the play button it morphs into a
pause button.
I'm not sure I buy the idea of buttons pressing in and popping out on 2D
interfaces. Colour might be a better indicator. Even something as
simple as the button becoming white instead of gray would make it clear
which button is active, and perhaps when paused the button could assume
a light gray shade.