The fundamental flaw for Metro apps
lack of a persistent Navigation Bar with at least a Home button. Too many apps you start navigating to content and before yo know it, you have to tap the Back button multiple times to get to the home screen. That is a UX failure. The new Windows Store app finally solves that by having a nav bar strip at the top, kudos Microsoft you finally got it. If you look at 99% of iOS apps they all have a nav bar of some type. I wonder why MS is not pushing this as a UI guideline.