There is lot of noise around this concept and yet there is still lot of ambiguity in the definition. Every one seems to agree Web2.0 is not a thing but a collection of things and it is the next generation of web as a platform offerings.
People tend to compare and show the contrast between earlier generation companies and tools vs Web2.0 companies. Like the difference between Netscape and Google, Ofoto and flickr etc., This article by O'reilly gives a good overview about Web2.0
Also, another often sited difference is the thing called blogs through which we are communicating. Blogs have mushroomed in the last couple of years and continues to grow. Personal web pages have been around for a while and I myself used to have my personal page in Tripod and Geocities. So, why are blogs different, popular and most importantly Web2.0? One perspective is that. It is the chronological ordering and the feedback mechanism along with RSS that has truly changed landscape of personal pages and its usability. For that matter, push technology has been there for a while, and I used to have screen savers that constantly pushed news and stuff to my machine. But then again, it was limited to few channels the provider gave as compared to RSS feed which anyone can create for their pages.
What's your take on Web2.0? How is it going to impact the software vendors like IBM, Microsoft, Oracle, SAP and others?