It appears that it offers a different approach to defining "routes" (in the MVC sense) using lambdas to identify relative paths, arguments, and the implementation of the response.
Ultimately, the framework's key benefit is its expressiveness. In ASP.NET MVC the RouteTable is in the global.asax and the implementation is in the Control. It appears that in NancyFx, this is the pattern that prevails:
Action["/path"] = args => { return your_implementation_here; }
Example implementation:
Get["/products"] = id => { return GetRepository().Products.Single( q => q.Id == id); };
Explanation: An HTTP Get to the relative endpoint '/products' with an argument of 'Id' will return a single product from the repository where the Id argument matches the product's Id.
Expressive and Concise.