According to the Wikipedia, Web2.0 "refers to a second generation of services available on the World Wide Web that lets people collaborate and share information online. In contrast to the first generation, Web 2.0 gives users an experience closer to desktop applications than the traditional static Web pages". Apart from the underlying technologies that allowed this (public web service APIs, Ajax or web syndication), web2.0 puts the emphasis on the social aspects of the world wide web. This component is connected to the "markets are conversations" concept from the Cluetrain manifesto that evolved in the "blogs are conversation" promoted by doc searls and others (see for instance this post). This has also been termed "l'Entrenet" by Daniel Kaplan.
But what is new here is rather the scale than the phenomenon. See this article by Thomas Erickson: The World-Wide-Web as social hypertext (communication of the ACm) in 1996: the authors admits that he does not pay a lot of attention to the Web (we're in 1995) and he had been converted to that:
The cause of my change of heart was the widespread appearance of personal pages. Personal pages are similar to informal resumes, except that in addition to professional material they often contain personal information. Hobbies, research interests, pets, professional publications, children, politics, friends, colleagues, all are grist for the personal pagestructed portrayal of a person. This insight leads me to characterize the Web as a social hypertext. The nodes—at least some of them—are becoming representations of people. And this, in turn, enables another critical feature to emerge: links from a personal page often point to socially salient pages. A common feature of the personal page is a list of pointers to “interesting people and places.” (...) This insight leads me to characterize the Web as a social hypertext. The nodes—at least some of them—are becoming representations of people. And this, in turn, enables another critical feature to emerge: links from a personal page often point to socially salient pages. A common feature of the personal page is a list of pointers to “interesting people and places.” (...) The transformation of the Web into a social hypertext has a number of interesting ramifications. Perhaps the most immediate and practical is that social hypertexts allow a fundamental shift in the way people search for information. Rather than composing queries for search engines or going to likely places to browse, (...) people can instead pose the question: Who would know? Or who would know someone who would know? Navigating from one personal page to another, we suddenly have a new sort of search strategy. (...) The same issues arise, mutatis mutandis, in sharing one’s own work. In short, the ability to find out what someone else is doing, without mutual knowledge of what’s happening, is a boon to both parties. This nonmutuality of knowledge is one of the characteristics that makes social hypertext different from more direct forms of communication
The article also discuss the fact that the web affords "presentation of self" and some goffmanian other issues that I won't comment here. The conclusion for that matter is great: "Although the Web may be just the latest fashion to sweep the Internet, if it turns out to be a medium that allows the construction, negotiation, and propagation of the styles of appearance we refer to as fashion, its impact may be profound indeed.",
Why do I blog this? because it's interesting to trace the previous trend we have today in early days of the web. The web as social hypertext is more or less connected to some web2.0 feature that we now have at a larger scale.