How do I power off my Mac with the Mavericks OS?
From Apple's support page, hold the power button for 1.5 seconds and let go to get the power dialog. Don't hold it too long though, as it only takes 5 seconds to turn off the system without a proper shutdown.

You can also get the dialog immediately by holding control and pressing either the eject key or power button, or use the Apple menu to access the three options directly.