One of the new features in OS X Mavericks is a slight change to the way the power button on your Mac is used. While pressing the button on a powered-down system will start it up as expected, with OS X running, instead of bringing up the power menu as it did in the past, the behavior now has three different modes.
The first mode is to sleep your Mac, where you press the button for about a second and then release it.
These changes can be useful for some people, but if you prefer the older approach, where pressing the power button immediately displays the power menu, then you can implement it.
To do this, you will need to configure a hidden defaults setting, so open the Terminal utility and run the following command:
defaults write com.apple.loginwindow PowerButtonSleepsSystem -bool no
When done, log out and back in to your system, and the changes should now be implemented to revert the classic behavior.
If you would like to revert this behavior, then run the following behavior to remove the altered setting.
defaults delete com.apple.loginwindow PowerButtonSleepsSystem