Don Hinchcliffe says the Seven Things Every Software Project Needs to Know About Ajax are:

1. The Browser Was Never Meant For Ajax.
2. You Won't Need As Many Web Services As You Think.
3. Ajax Is More Involved Than Traditional Web Design and Development.
4. Ajax Tooling and Components Are Still Emerging and There Is No Clear Leader Today.
5. Good Ajax Programmers are Hard to Find.
6. One Must Actively Address Ajax's Constraints of the Browser Model.
7. Ajax Is Only One Element of a Successful RIA Strategy.