AJAX is a new term, but XMLHttpRequest and related technologies have been around for a while. I know that, but the introduction of a single umbrella term nevertheless constitutes a tipping point which is forcing web development to move heavily in a certain direction. AJAX is only a name, but names can be tremendously important.
these are some points……..
Minimise traffic between browser and server so that the user feels the application is responsive.
Be clear on the interaction mode being used – regular HTML versus AJAX versus desktop application so that the user can predict what will happen next .. no surprises.
While avoiding confusion, borrow from conventions of HTML and desktop applications so that the user can rapidly learn how to use your application.