[DougWebb]

The drawback to validating the model in one place only, as shown, is that there is only one response for validation errors. Sure, it's got the details of the error that will vary, but it's ALWAYS sending a 400 error. Even the example of the 'usual' way includes a comment showing multiple possible responses:

    // return bad request, add errors, or redirect somewhere

An ActionFilterAttribute might be a good approach, but in a real application I'd want two things: to apply it explicitly at the controller or method level rather than globally, and to pass it arguments to tell it what to do when there is an error. One of the arguments should allow me to call a response handler method, even if the only way to do that is to pass a string with the method name and use reflection to find and invoke it.