It only takes mousing over the button to get an indicator that it can be interacted with. That combined with placement should be sufficient to communicate purpose.
The new Blackberry OS is like this with gestures. Sure, you can spend 20 minutes moving your finger(s) in every way imaginable to learn how to use it because there's no chrome (or hardware buttons), but it certainly doesn't instill confidence on the first use. Instead, it presents itself as a challenge: "learn how to use me, I dare you."
Not only that, it requires a mouse user also to hover over all possible elements to just know what's clickable. Kind of like some of the first graphical adventure games.
Or imagine a website where a link only turns blue and underlined as you mouseover it.
"Input" versus "Indicator" is a big problem in FlatUI that needs to be solved, there is no shared visual language yet to signify which is which, and it needs to be possible by just looking (and preferably other modalities as well)