Are you referring to extension methods? Or the "dynamic" keyword/type? I've only used the dynamic type a little bit, can you use it to solve the type vs representation problem he is referring to?
Yes. Basically it works by using the class hierarchy to define types. And w/ subtype polymorphism you get new types that are also classified under their base types. And with dynamic you now get the ability to join the "uber type" and be classified dynamically.