ASP.NET Web API Release Candidate gets several enhancements such as Json.NET as the default serializer, better testability of Http Message handlers, the IAPIExplorer API and more.
Notably, Json.NET now becomes the default serializer for Web API which means we don’t need to swap the default implementation any more – for instance, to be able to work with dates (Json.NET 4.5 uses the ISO 8601 date format by default). As a result, the redundant System.Json.dll has been removed. As Microsoft puts it -
Json.NET provides the flexibility and performance required for a modern web framework.
HttpClient now derives from a new HttpMessageInvoker and the latter can be used for unit testing Http Message handlers, rather than using former directly which are heavier. Also the IAPIExplorer API has now been included into the release bits. The complete list of changes are included in the release notes.
ASP.NET Web API is a WCF-alternative from Microsoft for building RESTful web services. The RC was released with the other ASP.NET web stack components such as ASP.NET web pages 2 and ASP.NET MVC 4.