yeah i think by rtm they will have to add a "close button" "charm" to the standard set for the times when you really do want the app to close. for example if the app is not working quite right , not totally broke but just has a problem and you do not want to re-start or go to the desktop to find taskmanager.
You don't have to go to the desktop to find taskmanager, just CAD then click Task Manager, it even opens over the Metro display.
But I have to heavily disagree with the idea of adding explicit Close functionality to Metro apps, because it misses the whole point of having system managed application lifecycles. As the user of a computer it shouldn't ever be my responsibility to manage system resources just to keep the computer happy, I paid for an operating system to do precisely that. Also, like providing synchronous API calls, it encourages application developers to be lazy; the process of saving state and documents gets left till application shutdown,rather than happening automatically.
It may not (currently) be ideal in VM situations, especially those which allow over-commiting RAM, but that's hardly a reason to get rid of it. VM authors will fix their behaviour and users running on physical hardware will get the best experience possible.