Microsoft has made available a document showing the new features in C# 4.0.
The new features in C# 4.0 fall into four groups:
Dynamic lookup allows you to write method, operator and indexer calls, property and field accesses, and even object invocations which bypass the C# static type checking and instead gets resolved at runtime.
Named and optional parameters
Parameters in C# can now be specified as optional by providing a default value for them in a member declaration. When the member is invoked, optional arguments can be omitted. Furthermore, any argument can be passed by parameter name instead of position.
COM specific interop features
Dynamic lookup as well as named and optional parameters both help making programming against COM less painful than today. On top of that, however, we are adding a number of other small features that further improve the interop experience.
It used to be that an IEnumerable
You can download a Word document written by Mads Torgersen with a little more info here: C# 4.0