The AjaxHandler component has upgraded major versions from 1.6 (CakePHP 1.3) to 2.0 (CakePHP 2). Since this is not a backwards compatible change, the 2.0 version can only be used in CakePHP 2. During the conversion process, many other features were refactored or removed, here is a quick round-up.
No more automatic data gathering
In the older versions, any request data was gathered and merged into the Controller::$data property within AjaxHandler::startup(). This is no longer the case as I highly suggest using the new CakeRequest class to manipulate data in your AJAX action. This class can be accessed at Controller::$request.
No more content type restrictions
Previously you could only respond() an action using JSON, HTML, XML or text. Now you can respond using any content type defined in CakeResponse. Automatic JSON/XML formatting is still enabled.
Removed the valid() method
The valid() method was a very abstract and useless method that simply checked that strings or integers were not empty. The AjaxHandler should not be dealing with any validation (low level ones even more so), so it was removed. Please integrate your own data validation.
Besides the major changes listed above, any private class properties were converted to protected visibility, to allow class extension and the RequestHandler was replaced with CakeRequest in most situations, ala is('ajax') compared to isAjax().
If you have any questions, let me know!