Don’t worry, the old functionality is still there, it’s just that Mavericks has re-programmed the power button slightly. Here’s what it now does:

Press and release: put the Mac to sleep.
Press and release with Control ^ key: display the ‘Shutdown’ dialog box.
Press and hold (release after 3 seconds): display the ‘Shutdown’ dialog box.
Press and hold (10 seconds): instant power off (aka “hard shutdown”).

🙂