A particularly overlooked and, for its simplicity, useful feature of ASP.NET AJAX is the WebMethod, which is a server-side method that can be called by client script. The .NET compiler automatically adds client-side methods that take care of the AJAX request, JSON encoding and, to a degree, data type conversion. Sure, you COULD do this using a web service and XmlHttpRequest - but this is a quick-and-easy way of AJAX-enabling standard web form pages.
Microsoft has a long history of misnamed and mis-marketed developer tools. My favorite example is .NET, a relatively simple concept complicated by an initial decision to attach the label to Microsoft's upcoming generation of servers (e.g., "Windows .NET Server 2003"). The latest blunder is ASP.NET AJAX, a useful suite of technologies only partially related to "AJAX" development. Unfortunately, as a result of this some of the more useful aspects of the technology have been overlooked.