An idea, When shutting down, logging out or rebooting the system, let's remove the OK and CANCEL buttons. Instead confirm your selection by clicking the "shutdown" button that is already selected and enter a message below that says "Shutting Down in X seconds, PRESS ESCAPE to CANCEL" Why? Because the extra buttons are redundant. We already give the icons the look of a button. We should use them as such.
You cannot make a dialog which can only be operated by keyboard. Imagine you only have touch and mis clicked. The buttons are unfortunately essential for a consistent user experience for all input device categories.