I was totally lost on this as well. Although they expect Windows 8 (w8 for short, heh) to be installed on instant-on devices I guess, so shutting down isn't called for except for a hard boot with the power button. It's actually located in 2 places: Start > Settings > Power > Shut down/Restart OR Log off > enter login screen (slide up to activate, arrgh) > Power Icon > Shut down/Restart.
Who thought of this as the epitome of usability for a regular PC should be tarred, feathered, skinned, quartered and shot. Or made to use it him/herself on a regular PC for an extended period of time.