This Wiki article pretty much sums up web 2.0 for me. Basically in my mind it is a way of lumping together a few social trends that hit web sites at roughly the same time (through the emergence of new technologies such as AJAX).
The internet cannot be versioned off the way that other software can as it is continually evolving as users become more demanding and programmers become more inventive and the backbone architecture improves (I'm thinking broadband here).
Think back to the first web site you ever saw; white background, black text, minimal images, naff-all functionality. This then evolved to include more and more interactivity and functionality, peaking with the .com boom (and bust) in the late 90's and the massive amount of hideous flash-based sites
. Now we're getting back to a stage where the users are dictating what they want to see and do, hence the boom of social networking sites and more bespoke functionality. People are social creatures and also have very little time or inclination to be bull$hitted to. If I want to buy a specific cd, I want to do it as quick and cleanly as possible. However, if I'm browsing for something new then I may want to interact with fans, listen to clips, see what other people who bought stuff I already own have bought, etc.
I'll have a read of that article later, although I suspect 'web 3.0' will just involve a greater convergance of internet-ready technologies and that the average user won't see a difference, but hey anything that keeps me in work is all good! :)