I'm pretty sure Nancy predates the Web API by a decent amount of time. So it wasn't "already done" when Nancy was written.

They both have differences, and are equally valid choices. Aside from anything else, I give Nancy time because I'd love to see more open source independent development on the .NET platform.

skrowl, can you quantify your arguments as to why you think Web API is better? In what way? What do you not like about Nancy, where does it fall short?

How is anything WebAPI does, better?

Its routing is rubbish - waiting for V2 for fixed routing (AttributeRouting support so its not hidden away)

Its terrible to extend.

Its slower than Nancy.

Its hard to test out of the box.

Have you used Nancy? When you have then you can say WebAPI is better. Until then, keep your MS Fanboy comments to yourself