Apple changed the behavior of the Power button in 10.9 Mavericks.
Press the button once, and it puts your display to sleep. Press and hold it for a second or two, and the Shutdown / Restart / Sleep dialog appears. Press and hold it for even longer, and your Mac gets completely powered off.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20131025154203251