The Metro design language is one of the key strengths of Windows Phone. With the goal of providing a familiar interface model to the user, over both the built in and third party applications, it's not only a guide on how your applications should look, but also how to lay out screens, how to guide users through your app, making it feel comfortable and predictable to the users.

Paul Laberge looks at this idea of "application flow" on his MSDN Blog today, and how the process can help make developers application better:

Even before you do layouts for each screen of your app, chances are you are charting out the navigation of your app.  Which screen goes where, how to determine under what conditions the user may go to the next step, etc.  It certainly sounds trivial as it’s written, but in truth it’s one of the hardest tasks to do right in app design.

The flow of your app very much defines its character, more so than any screen.

Metro navigation guidance comes from 5 separate areas, each of which are related but focus on separate components of application flow.

He talks about those five areas (hub and spoke model, trust the hardware, avoid trap loops and phantom pages, be predictable, and offer integrated experiences), expanding on why they work this way and why you should be implementing them, before reminding you that an app that feels like Windows Phone is going feel natural and 'right' to the users downloading and using it.

It's a good grounding on Metro, and worth reading.