It first goes into the motivations for building a new Phone OS and what vision they had for it. Then it goes into the different development features and how it connects to the vision.
For instance, one of the objectives was to have the phone being customizable. One of the main way to do that is to install applications & games. Now, for this to really work, the application model needs to be predictable so that a given game wouldn’t degrade the entire experience. This is why they went with a managed application model, ie .NET (via Silverlight and XNA): applications are verified prior to execution.
Very nice introduction to a promising product.