I don't agree with the term Web 2.0 at all because there is no global change happening to the Internet.
Basically Web 2.0 was just a term popularized by O'Reilly which he thought should be used to describe a website with improved functionality and a dynamic platform to improve websites and increase the standard of website that we as visitors expect.
So basically, what they meant by that is improved websites. If you wish to take what they said to mean improved coding then yes Ajax falls into that category, but so does CSS, Validated XHTML and RSS. O'Reilly saw these "new" technologys as an improvement and a step in the right direction so he classified them as "Web 2.0" technological advances.
The idea is to improve web-based stanards, create a common goal for websites to be informative and well presented, but in turn lose their uniqueness. However I do agree with some things he mentioned just as browsers with standards i.e. all browsers will have to adhere to standards set out by a governing body to reduce problems in Firefox that don't occur in IE or the other way round.