Yes, metro apps are fullscreen. No menu bar or anything like that. So if you have some background tasks you won't be able to see if they're finished...
And apps can be suspended or closed. Suspended is something like 'sleep mode'. They're still in your alt-tab menu (you can directly launch them).
And regarding the last question: yes: metro is the default. When you boot up Windows you will see metro. And as far as I know there is no longer the option to remove it. Every time you need something you will return back to the metro start screen.
Apps with background tasks are generally supposed to pop up a notification when they're done (the APIs for the new notification popups are available for use by desktop apps too btw, although admittedly they probably won't be widely implemented for a while if ever).
And apps can be suspended or closed. Suspended is something like 'sleep mode'. They're still in your alt-tab menu (you can directly launch them).
And regarding the last question: yes: metro is the default. When you boot up Windows you will see metro. And as far as I know there is no longer the option to remove it. Every time you need something you will return back to the metro start screen.