Since upgrading to RC for WebAPI I'm having some real odd issue when calling POST on my WebAPI.
I've even gone back to the basic version generated on new project. So:
public void Post(string value)
Since you have only one parameter, you could try decorating it with the [FromBody] attribute, or change the method to accept a DTO with value as a property, as I suggested here: MVC4 RC WebApi parameter binding
UPDATE: The official ASP.NET site was updated today with an excellent explanation: http://www.asp.net/web-api/overview/working-with-http/sending-html-form-data,-part-1
In a nutshell, when sending a single simple type in the body, send the just the value prefixed with an equal sign (=), e.g. body: