I have seen many developer tip series or tutorials for Windows 8 that either leave out the design aspect of the platform or they save it until the very end. This is unfortunate because design is a strong focus of Windows 8. Many developers feel weird reading design-related tips in a series that is supposed to be about development. I've got news for you though. If you don't have a designer for your app and you aren't planning on getting one, you're officially a designer now. Now that we're on the same page, here are some resources that can make sure you're on the right path with your application design.
(Previous tips can be found here.)
Microsoft's design documentation
For those that like to get their information "from the horse's mouth," Microsoft's documentation at design.windows.com covers just about every aspect of Windows 8 design in varying levels of detail. Here you'll find tips for planning your app, some inspiration, and some case studies on how to bring apps from other environments and platforms into the Windows Store. You will also find all of the expected platform-specific guidance. They even have design assets that could help your designer (if you have one!) by giving them some things to work with inside of Photoshop.
If you're the type of person that learns best from books, you'll be happy to learn that there is one on the way for this topic. My book, Designing for Windows 8, will be released by Apress on January 30, 2013. It is currently in the Apress Alpha program so if you are interested in getting an early jump on reading it you can purchase it now. The remaining chapters will be added soon. The book picks up on topics not covered in Microsoft's design guidance such as more advanced planning strategies, prototyping tips, and platform considerations. It has been written such that developers and designers alike can learn the information.
Here is a recommended list of things to watch if you are a visual learner:
There are more than are worth mentioning here. This is a very hot topic. I have articles on the subject here as well as at CodeSnack but many others have also written on the topic. You may need to search for "Metro" or "Metro style" to find some of the older stuff. "Microsoft design" or "Windows design" with or without the word 'style' or 'language' at the end are the more recent terms you can search to find more.
Windows 8 is a platform with strong design guidance. This is good news because Windows application design has been the wild, wild west for far too long. We have grown accustomed to throwing every feature into the app wherever we want on the screen and that all has to change in Windows 8. I hope the material here serves as a good starting point on your journey to creating a great Windows Store application. Remember: if you don't have a designer, you are the designer.
If you have any questions or comments, please comment below or find me on Twitter @brentschooley. You can also email me at firstname.lastname@example.org.