Jim Rapoza at eWeek has an article up called What Web 2.0 Means to You:
Basically, Web 2.0 is a marketing term O'Reilly Media coined a few years ago when it was planning a conference to cover significant Web changes.
But the term has gained a lot of traction, and one sees it not only in discussions of Web technologies but also in product literature as, predictably, many vendors start to position products as "Web 2.0-ready."
Maybe I am just really cynical, but I'm kinda of turned off by the term Web 2.0. To me, it sounds like too much marketing fluff and not enough real stuff (Look, I made a rhyme!). I understand that blogs, RSS, and Ajax, to name a few, are the new hot technologies now, but is that really enough to start calling everything 2.0.
Jim addresses some of this in his article, which you should definitely read. I do like what he says at the end:
Tim says that a real Web 2.0 product spreads through word of mouth and grass-roots enthusiasm, and if a product is advertising itself as a Web 2.0 product, then it probably isn't
Amen!