If you havn't yet watched Ander's video on C# 4.0, watch it here, right now. http://channel9.msdn.com/pdc2008/TL16/
For me the most exciting part is the new dynamic dispatching features in C# 4.0. Dynamic dispatching provides a great way for resolving method calls at runtime. The new dynamic keyword can be used to create dynamic objects.
Watch out this space soon for a couple of practical examples. Dynamic dispatching has hell lot of possibilities, so that you can have wrappers around anything to execute your calls!!
Ander's video also covers PLINQ and a heart warming demo of C# Read-Eval-Print loop using Managed C# Compiler.