A few considerations when designing applications using AJAX Technologies
Progress & Activity Indication – must provide controls that provide users with progress and or activity indication. Can’t always rely on the availability of the browsers progress bar to indicate a use case within your application is being processed. Even with AJAX there are processes that can take a while and the user must receive feedback if there is a noticeable wait between transactions.
The Back Button – need to provide a back button. Studies have shown that many users rely on the back button for navigation. With AJAX you can’t completely rely on the browser back button for your application’s navigation. Often within the browser there is no page to return to or page history to navigate to.
Contextual Help – Provide contextual help to aid users and as a technique for informing users on many of the features, built in short cuts and other rich-UI features that in your application. Contextual help is help within the context of a user action rather that through documentation and or a help section for example tool tips. Using AJAX technology you can build additional and richer contextual help options through out your applications.