Web API is the first real alternative to WCF that .NET developers have seen in the last six years. Until now emerging trends such as JSON were merely exposed as WCF extensions. With Web API, developers can leave the WCF abstraction and start working with the underlying HTTP stack.
Web API is built on top of the ASP.NET stack and shares many of the features found in ASP.NET MVC. For example, it fully supports MVC-style Routes and Filters. Filters are especially useful for authorization and exception handling.
In order to reduce the amount of mapping code needed, Web API supports the same Model binding and validation used by MVC (and soon to be released NET Web Forms 4.5). The flip side of this is the content negotiation features. Web API automatically supports XML and JSON, but the developer can add their own formats as well. The client determines which format(s) it can accept and includes them in the request header.
Making your API queryable is surprisingly easy. Merely have the service method return IQueryable<T> (e.g. from an ORM) and it Web API will automatically enable OData query conventions.
ASP.NET Web API can be self-hosted or run inside IIS.
