Paul M. Jones has started an interesting discussion rethinking the MVC pattern as applied to the web, which he has dubbed Action-Domain-Responder (ADR). If you haven't given it a read yet, click the link and do that; this page will still be sitting here waiting when you return. I agree with a ton of it - heck, I've contributed to it a fair bit via conversations with Paul. But there's been one thing nagging at me for a bit now, and I was finally able to put it into words recently. Controllers - Actions in ADR - can be explained as facades.
Matthew starts off by defining the Facade design pattern with a quote from the infamous "Gang of Four" book: simply put, a simplified interface to a complex system. He provides a basic example of a facade that wraps some common steps for inserting and logging data with this kind of simplified interface. He applies this to the ADR pattern's controllers, pointing out that it handles a few complex steps "behind the scenes" common to marshaling and managing the request.
For me, thinking of Controllers and Actions as Facades has an additional benefit: it describes rather complex architectural patterns in terms of basic design patterns. I find the more I can reduce the complexity of a definition, the more likely I will understand and use it correctly.