It is fantastic to see that Mono is keeping up with the CLR / .Net evolution. They just reached the feature complete state for their C# 4.0 compiler, as reported by Marak Safar from the Mono compiler team.
Optional parameters, named arguments, dynamic binding and of course covariance and contravariance. I’m especially excited about the last one as it will help a lot with complex generics-based frameworks. That said, dynamic binding remains the most general purpose feature in this version.