Web 2.0 is perceived as the new way that the web works. Exactly what is web 2.0 nobody really seems to define exactly; Is it the collaborative aspect of the web?, and if so, does old web sites that allowed users to add their comments were web 2.0?
Is it be that sites that use the Ajax technology are a web 2.0 sites, or are they just sites with an improved user experience? as far as I see it, web 2.0 is a revolution in the current technology.
Through out the history, any technological revolution was based upon several pillars, which can be summarized to these: the technological advance pillar, the business model pillar and the platform pillar.
The technological advance pillar is basically the answer to the question "what's new in this technology?", The business model pillar is the answer to the question "how would somebody make profit out of it?" and the platform pillar is the answer to the question "what would drive this technology forward in order to allow people to use it?"
Think of past technological revolutions, and see that they had to stand on these pillars. It may be the industrial revolution or the antibiotics revolution, still, all these questions had to be answered.
So how does web 2.0 answers these questions? the technology is Ajax, the business model is the long tail and collaboration and the the platform is the web.
So, what defines a site to be a web 2.0 or not, in my opinion, if you make money out of it, not just by displaying your merchandise and selling it (and it could be either physical goods like books or virtual like advertising or information), but also allow the users to interact and influence the system, hopefully using Ajax to improve the site's user experience, then this is a web 2.0 site, otherwise, it is not.