AJAX has been a great leap forward in Web applications. The problem however is that many sites have now gone overboard with it shifting the burden of processing to the client browsers. On the surface this is a welcome change, but there is a drawback when applications rely too heavily on the client side. Many sites that employ AJAX and DHTML end up with slow and unresponsive pages. While bits and pieces of a page are being loaded and formatted from various sources, users must wait until the whole page is rendered before they can proceed.
In many cases, those pages go into a frozen state until all components are properly loaded and formatted and many don't respond to user inputs while completing their tasks. The problem is not widespread yet, but as many sites begin to deploy AJAX, those Web pages will begin to crawl wearing users' patience thin. One notable example is Microsoft live. The site looks great, but the site's home page takes an inordinately long time load. At times I have just abandoned the page while it attempts to load its various parts.
AJAX is a great tool for making Web pages more interactive and flexible. But there is a point where too much technology can hinder usage instead of helping usability. Sites shouldn't just throw in new technologies indiscriminately hoping that users would come in droves. In the end, if a site proves too slow and unresponsive, users would just get turned off, no matter how bleeding-edge.