Web 2.0 rundown
Web 2.0 is a problematic term, because it is not so much a different product as it is a different approach to web content and as O'Reilly puts it:

"the Web 2.0 meme has become so widespread that companies are now pasting it on as a marketing buzzword, with no real understanding of just what it means. The question is particularly difficult because many of those buzzword-addicted startups are definitely not Web 2.0, while some of the applications we identified as Web 2.0, like Napster and BitTorrent, are not even properly web applications!"

Web 2.0 has become a marketing term meaning "New and fresh" - even if it is NOT new and NOT fresh. The technology behind Web 2.0 has been around for ages, but it isn't until recently that clever programmers created tools for using it in a structured development process.
Managing the development of a Web 2.0 application is more than just programming - but it is also more than just a way of thinking. The way of thinking has to be supported by the code and developing with AJAX can easily go out of bounds if not done carefully - and end up in excerpts on websites such as www.thedailywtf.com

In conclusion

Web 2.0 with AJAX and similar technologies applied is a great way to utilise the emerging possibilities on the Internet, but it has to be done with care and not just because a CEO says that everyone else is doing it.

The best implementations of Web 2.0 is those that don't make you think that it is Web 2.0 - it's just good.