It was recently brought to my attention that knowing the design guidelines prior to building an application saves the developer a lot of time, since incorporating the design guidelines and certification requirements makes the application acceptance process faster. Here are some hints that I stumbled upon:
Navigation, frames and pages
Tiles and tile notification
Canvas/Grid for layout
Panorama control/pivot considerations
Also, another helpful design cheat sheet that Nathalie BELVAL has put together this handy card for designing and developing apps for Windows Phone 7.x and Windows Phone 8. It contains all the information you might need for:
Download it, print it, love it.