My personal view is that Web 2 stuff is best used (in the first instance anyway) to improve the things that people do all the time and that require server hits. Think of gmail - it doesn't do anything new, it's just much nicer to use.
Another view that I have is that frameworks like OACS have many pieces of functionality that can be upgraded to Web 2 that will enhance existing sites for free.
First example is the form builder. It should be feasable to make form builder use 2.0 stuff to make all form validation happen in-situ. List builder is another case where some live functionality could be added that will improve the user experience.
First, though, you want a 2.0 enabling package with tcl and javascript helpers so that you're not re-inventing the wheel in every package.
