For me, it is the first time that finally we have something really new in term of UI that is compliant with most of the modern architectures that are server side Java based. You can argue that Flex, OpenLazlo, ..., are very good and powerful, but first of all they use a proprietary plugin and really integrated with the development approach that most of the Java developers are currently using (MVC based on Struts, JSF or others). What get me very excited about Ajax is the fact that finally we can see Web Development giving an easy and standard/open way of creating rich client.
If you search around Ajax your will see that more and more framework are providing integration of Ajax technologies into their solution. What I see as the biggest move is the adoption of Ajax by JavaServer Faces, and simply because it is the J2EE standard for Java Web Development. I am sure that lot of you will say that Tapestry,ROR and other existing frameworks are providing the integration now, so why bother with JSF... Hmmm, I will say that all the current solution as still geek oriented and really focus on the core developers... What I see with JSF/Ajax integration is finally enterprise developers, that are used to 4GL development tools, (where the most important part is to develop business logic not a nice framework or a new set of libraries) will be able to develop Web Applications based on components that are smart enough to give the usability of a desktop application.