Sure, HTTP status codes should be used. But they aren't enough to communicate application-specific messages from an app backend to an AJAX front-end. For example, AJAX on the browser might submit a form and the server returns a 422 (HTTP validation error status code). How will you be able to inform the user that the "email address must contain @" or "foobar is not an acceptable name"? You need to consume some JSON that has a payload of application-specific status messages.

I don't know if this proposal is optimal (smarter guys than me can decide that). But I know that a standard way of communicating application-specific errors would be a big step forward.

A year ago I searched for an adequate in-place editing plugin for Rails that didn't require major surgery to provide error messages to the user. I was surprised at the complexity required to handle a simple error condition. Usually Rails has all the goodness baked in but it seems application-specific status messages with JSON are out of scope. I asked "How to show server errors in Rails with JSON and jQuery" [1] on Stack Overflow and it's received more views than any other question I've ever asked.

[1] http://stackoverflow.com/questions/2489567/how-to-show-serve...