Not having an API is becoming like not having a website, but the interface has got to be easy for outside developers to work with.
The industry group offers guidelines for user interfaces, privacy, stability and more.
Take advantage of MVC, MVP and MVVM Design Patterns to build applications that are easier to test and maintain.
Rick Barraza from Channel9 has a really good article on the new asynchronous pattern in WinRT for handling requests to and from the server, as well as putting together the Data Model for our app. If this topic is new to you check out this quick start on how to call asynchronous APIs in .NET.