With OS X Mavericks, Apple changes the behaviour of the power button on your Mac.
Tap the power button once will now put your Mac to sleep. Press the power button again to wake your Mac. The duration is similar to pressing the Space bar or Return key to wake your Mac.
Press and hold the power button for 1-2 seconds will bring up the shutdown dialog:
If your Mac has a Media Eject key (), you can also access the shutdown dialog by pressing Control and Media Eject keys simultaneously.
Press and hold the power button for 5 seconds to force the Mac to shut down completely. Forcing your Mac to turn off this way may lose unsaved documents. Use this method only when your Mac is unresponsive for an extended period of time.