Extending Game Studio Express
The power of Game Studio Express is achieved with the use of managed code and visual tools such as Controls that take large, complex parts of game construction out of the equation. What will make Game Studio Express popular is if people actually write and contribute code to the XNA community.
There are a number of companies already doing this, which is amazing because the XNA Game Studio has been out for only two months at the time of this writing. One of the best places to check for open source solutions you can use to extend Game Studio Express is CodePlex.com, where you can find a number of "work-in-progress" projects.