Disclaimer: I'm not a supporter of NancyFx :)
I'm in the process of evaluating if I should go with NancyFx or with ASP.NET Web API for the REST part of a project.
Apart from simplicity and expressiveness (which do have a value on their own, I think) already mentioned by GlennFerrieLive, I think I've grasped another couple of nice bits:
It's easy to perform operations before and after any API request processing, in a kind of an Aspect Oriented way, so to say.
By default the framework takes care of the accepted returned type, so it will appropriately convert output in JSON, XML, ...
Lambdas implementing requests do not return actual filled data, but still in the form of a query. After that it's still possible to easily add filtering, sorting, and other operations before actually executing the query, hitting the DB, and returning the actual data.
They have somehow wrapped the HttpRequest and made available to the developer an equivalent of that, with the difference that this new object is injected and you can of course substitute it with a mock ... So easier and cleaner testing.
Maybe some of those (all?) are already available in ASP.NET Web API and with the same ease, I don't know for sure.
HTH
