The disadvantages of AJAX are:

•   Search engines would not be able to index an AJAX application.
•   The server information can not be accessed within AJAX.
•   AJAX is not well integrated with any browser.
•   ActiveX requests are enabled only in IE 5 and IE6
•   Data of all requests is URL-encoded, which increases the size of the request.

You have said that,"ActiveX requests are enabled only in IE 5 and IE6"...If some plugins are not found in a web application,now a days, all the browsers are requesting to install the plugins...Is plugins are made of ActiveX controls???