How To 'Go Back'
If you are playing music, it is very obvious, the back arrow in the upper left.
If you are navigating most other features like Marketplace, Social, Apps, Settings, it is not so obvious and more then one journalist has commented on this.
In actuality it is very simple to 'Go Back'.
I have read criticisms of the large page titles, in that the titles do not fit on the screen. Well it is very obvious as to why they did this. Those titles aren't really there to remind you where you are, they are your 'Back' button.
Try it. Tap MARKETPLACE. See the giant letters at the top of the marketplace page, tap them and you will be taken back one page/screen.