Async, lambdas, extension methods, linq and "the dynamic type" (late binding). Yes, many similar but older implementations for these exist, I know, but in C# they work in harmony - and that's a bit hard to explain for me =)

And more syntactic sugar than is good for your teeth :) (I'd also want to add generics to the list, but it's over 10 years old, still not many languages that implement it so well)

Type inference.

Properties, lambdas (thought they have it in Java 8, but it is still too late) to start with.