Of course we are missing the Start button in Windows 8 and that means, there is no simple way to shutdown Windows 8.
To shutdown Windows 8, we open Charms bar by hitting WIN+C and select Settings.
Then select Power button.
It will launch a menu bar with Sleep, Shut down, and Restart menu items.
Select on Shut down will shut down Windows 8.
Alternatively, you can hit CTRL+ALT+DEL and select the Shut down button at the bottom right corner.
I hope to see the Start button in Windows 8 some point.