With Windows 8.1 Update 1, Microsoft plans to bring a prominent Shutdown button to the Start screen. It is a useful addition because you no longer have to swipe from the right, then use the Settings Charm, and then click Power to shut down. However, should you want to hide this button to avoid clutter, there is a simple registry tweak which can be used to hide the Shutdown button from the Start screen.
Note: in Windows 8.1 Update 1 RTM (Spring Update/Feature Pack) MS did a change for value name and location. I've updated this article with the actual information.