Suprised that reflection wasn't really mentioned outside of the concept of generics. Reflection in .NET (and C#) is extremely powerful and useful.

All those other things that people have already commented on like Lambdas, expression trees, events and delegates, are incredibly nice to have.

Here's the first thing I write when I start a Java project, followed directly by an abstract class based on it:

public interface ICallback<T> { public void Invoke(T state); }