Update 13.07.2009: There is a separate post for security transparency.
With the eminent release of a beta release for Visual Studio 2010 next month, I thought it would be nice to have learning resources for the new features introduced in .NET 4.0 and especially in C# 4.0.
Covariance and contravariance
Named and optional parameters
Thread pool engine
There is a separate post for security transparency.