Since OS X 10.9 mavericks, the behavior of the power button has changed. Now, when you quickly press this button, it will OS X be placed in standby mode. With the following command you can use the power button function rollback to previous versions of OS X.
defaults write com.apple.loginwindow PowerButtonSleepsSystem -bool no
To revert to the original settings:
defaults write com.apple.loginwindow PowerButtonSleepsSystem -bool yes