If you want to use the [WebGet]/[WebInvoke] programming model in WCF, you need an endpoint with the right binding and the right endpoint behavior.

The binding is the out-of-the-box WebHttpBinding. The endpoint behavior can be either WebHttpBehavior or WebScriptEnablingBehavior.

What's the difference between the two?

The WebHttpBehavior is a general-purpose behavior that supports UriTemplate dispatch and POX/JSON/byte stream formats on the wire. Use it for general purpose HTTP/REST/Web-Style services.

The WebScriptEnablingBehavior is a "profile" of the WebHttpBehavior functionality designed specifically for interop with ASP.NET AJAX clients. It adds in some AJAX-isms like the ability to automatically generate ASP.NET AJAX client proxies.