I find NancyFX really interesting, but I can't really see a reason to stray away from Web API2 besides Mono support. Any specific reasons you prefer Nancy?
I've been wondering this, too. I've been using MVC since version 2 and am very comfortable with it and Web API, but a startup I've been doing side-work for is using Nancy. Nancy does seem lighter weight, and includes TinyIoC for dependency injection out-of-the-box, but I too don't see a real killer use case for Nancy.
It's nice to serve webapi's and serverside pages from the same class. I like the content negotiation in Nancy.
Also the built-in testing features are nice, it is easy to test views too.
Mono support is important for us and I prefer the syntax. It's also handy to be able to serve up ancillary pages like documentation alongside the API.
I agree with you, but maybe because it's lightweight? It's pretty damn simple.