New C# Features in the .NET Framework 4

1)Covariance and Contravariance
2)Dynamic Dispatch
3)Dynamic and the DLR
4)Named Arguments and Optional Parameters
5)Indexed Properties
6)Omitting the Ref Keyword at COM Call Sites
7)Embedding COM Interop Types
http://msdn.microsoft.com/en-us/magazine/ff796223.aspx