I don't know why they don't offer a similar experience to wp7 mango when switching applications. If you hold or double tap the window key it would bring up a scrollable list of the opened applications. They could then easily put a close button in the top right hand corner.

This solves 2 problems as currently to switch between apps you have to swipe endless amount of times to find the application you are interested in. It will also be quite a familiar experience to people using iOS.