I really hate the buzzword Web 2.0. It doesn't really mean anything, but (like most marketing) it's catchy enough for people to repeat it verbatim and actually apply anything that they mean. Ajax? Yeah, that's Web 2.0. Semantic web? SOA? Heck, give Sun half a chance and they'll release Java 6 1.6.0_1_b79_web_2_0.
I think that UserFriendly sums it up the best; you should read their take on Web 2.0