JQuery/WCF without ASP.NET AJAX:

At first glance there are three problems with your code:

1: you should use the ServiceKnownTypeAttribute to specify known types when exposing only base types in your operation contracts:

[ServiceContract(Namespace = "yyyWCF")]     
public interface IClientBroker
{
    [OperationContract]
    [ServiceKnownType(typeof(Client))]
    [WebInvoke(
        Method="GET",
        BodyStyle=WebMessageBodyStyle.WrappedRequest,
        ResponseFormat=WebMessageFormat.Json)]
    IClient GetClientJson(int clientId);

}

2: You should use WebMessageBodyStyle.WrappedRequest instead of WebMessageBodyStyle.Wrapped because the latter is not compatible with WebScriptServiceHostFactory.

3: IMHO using Method="GET" would be more RESTful for a method called GetClientJson than Method="POST"

Another advice I could give you when working with WCF services is to use SvcTraceViewer.exe bundled with Visual Studio. It is a great tool for debugging purposes. All you need is to add the following section to your app/web.config:

  <system.diagnostics>
    <sources>
      <source name="System.ServiceModel"
              switchValue="Information, ActivityTracing"
              propagateActivity="true">
        <listeners>
          <add name="sdt"
              type="System.Diagnostics.XmlWriterTraceListener"
              initializeData= "WcfDetailTrace.e2e" />
        </listeners>
      </source>
    </sources>
  </system.diagnostics>

Then invoke the web method and WcfDetailTrace.e2e file will be generated in your web site root directory. Next open this file with SvcTraceViewer.exe and you will see lots of useful information. For example it could say:

Cannot serialize parameter of type 'MyNamespace.Client' (for operation 'GetClientJson', contract 'IClientBroker') because it is not the exact type 'MyNamespace.IClient' in the method signature and is not in the known types collection. In order to serialize the parameter, add the type to the known types collection for the operation using ServiceKnownTypeAttribute.

Of course you should not forget commenting this section before going into production or you might end up with some pretty big files.

I am 99% sure you cant return an interface. I dont think Interfaces are serializable.

check out this thread