From Ajax Patterns
Specific goals closely related to Ajax application design.
Be careful about conserving resources in the server system, the browser system, and the connection between them. Resources such as bandwidth and system load have always been an issue, and Ajax pulls the lever even higher. With the possibility of frequent round trips, consideration must be made as to the transmissions' size and value to the task at hand.
Ajax applications should consider specialised needs of users with particular disabilities, of different ages, backgrounds, and so on, as well as different browsing devices such as mobile phones. The importance of these factors will depend largely on the particular application being developed.
Ajax applications can be intrusive from a privacy perspective as it's possible to send the server frequent updates about the user's behaviour. This information can improve the user experience, and may be valuable to the server-side party, but should be balanced against privacy concerns.
Design principles, more specific than the overall goals, but not patterns as they do not cover concrete design features.
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.
Avoid distractions such as gratuitous animations so that the user can focus on the important things.
While avoiding confusion, borrow from conventions of HTML and desktop applications so that the user can rapidly learn how to use your application.
Stick with Ajax wherever possible - just say no to entire page downloads so that the user’s experience is consistent.
Relevance to Design Patterns
Some (and eventually all?) of these principles inform the Ajax patterns. The patterns are about trading off among the principles, and also about resolving conflict between the needs of usability and other practical constraints, such as ease-of-development.