Windows 8 still supports the command line feature to shut it down. You can just press the Win+R button to open the run dialog box and type the command
shutdown /s /f /t 0
/s for shut it down
/f for forcefully close any application and shut it down immediately
/t for time in milliseconds, without delay
/r for reboot, replace the /s
There are other ways to shut it down easily
You can shut down Windows 8 from the login screen after signing out. To sign out, open the Start screen (press the Windows key or move your mouse to the bottom left corner of your screen and click), click your name, and select Sign Out. Click the power button at the bottom right corner of the screen and select Shut down.
You can shut down Windows directly from the desktop. First, ensure the desktop is focused by clicking your desktop background, and then press Alt+F4 (as we all know if you press this key combination while a program is focused, that program will close.) Select Shut Down in the dialog that appears.
Update: You can also add
shutdown, restart, log off, switch user to the desktop right click(context menu) menu. Just download this registry file and run the
add-menu.reg file and it immediate add the menu to the right click menu.
Information from this source.