I'd argue that MS did a great job with flat design in Windows Phone 7 because they were willing to sacrifice aesthetics for consistency. Everything on WP7 was

I'd argue that MS did a great job with flat design in Windows Phone 7 because they were willing to sacrifice aesthetics for consistency. Everything on WP7 was stark black backgrounds, which made it really obvious what was a button - a colored box was always clickable. The moment you let graphic designers in who care about making things look good, flat design falls apart because they want to change it up. Flat works if you let the designers construct the design language and then never let them near the actual individual UIs and get tempted to deviate from their original language.