Web 3.0 is already here.
It is Flash/Flex/WPFe.
AJAX is good for certain applications, but it doesn't have the horsepower to provide the next generation user experience to web users.
Development experience - You do not have to practice voodoo to write cross browser UI anymore.
Customers - You don't have to be scared to click that link because you never know which link will take the server roundtrip.
The main issue with WPF/e so far is it's unsuitable to create application like Flex is.
Let us wait and see whether Adobe or Microsoft executes best this vision!