We've created the look of a button in it's regular state, but a 3D button like that is begging for a "pushed" state. What we do is add a darker color to the button on :hover and :focus. Then for :active (pushed), we remove some of the box-shadows and nudge it down and to the right with relative positioning. It's as simple as that, the button now has a really obvious and satisfying experience when you click it.
One that that springs to mind: when you adjust relative positioning with top/left/bottom/right values like that, you have to be careful in the future if you need to absolutely position the element. You might get some crazy jumps going on, since top/left/bottom/right means something far different with absolute positioning than it does with relative positioning.