So, I learned a very hard lesson that the WCFTestClient is not meant to test/debug/develop WCF Web Services with JSON. There is an ability to customize the client configuration file within WCFTestClient to bind web services, but maybe that's for future-proofing when and if WADL becomes more commonplace.
How can I create a C# application which will consume this JSON-enabled WCF Service? Or, must I test it in the browser?
Thank you.
I'm assuming you are creating these services by using the WebInvoke/WebGet attributes on your method signatures. You can import these services just like you would any other service into a C# project, however VS doesn't add the additional attributes in the signature to the created code once you import them. You can show all files, and go into your service reference, under Reference.svcmap, into the Reference.cs file and add the attribute to the signature in the interface for your service, they will then allow you to send and/or receive them with JSON.
For example make sure they both your service interface and the imported interface have:
[WebInvoke(ResponseFormat = WebMessageFormat.Json, RequestFormat = WebMessageFormat.Json, BodyStyle = WebMessageBodyStyle.Bare)] 
GETverb. For more advanced and more powerful testing of REST services, use Fiddler - free and very very powerful!