If you haven't started WinRT development in earnest yet, you have only a partial notion of what it is and how it works. If you've been working with it as a newer developer, you may not be aware of the overall context of the system. If you are a more seasoned developer, primarily accustomed to .NET, you may not fully understand how it fits together in the broader Windows ecosystem.
WinRT is a brand new paradigm for developing apps due to its rethinking of the environment, but it's not a totally new product either. Read this article by Peter Bright/Ars Technica to get an amazingly in-depth (and correspondingly long!) take on the evolution of Windows and how it leads to WinRT. It's a great read!
Link: Turning to the past to power Windows' future