As companies and individuals continue to sort out just what the big deal about Ajax is and how it can help them, they need to also look at some of the things they need to consider about its implementation into their site.
FrontForge has listed to help to get started.
The list is:
1. Back button, history, and bookmarks
2. beyond widgets: standard and custom behaviors
3. single-page interface
4. productivity and maintainability
5. client-server
6. XPath targeting
7. comprehensive event model
8. state and the observer pattern
The detailed explaination can be read in : Top 8 Ajax evaluation criteria