What Can Web 1.5 Teach Us About Web 2.5?

Lately there's been lots of buzz and hype about "Web 2.0" (See here for a popular essay by Tim O'Reilly that explains what that generally means.)  In trying to think about how relevant all this might be for clients, I've been thinking a lot lately about "past as prologue".  More specifically, how did "Web 1.0" play out over 2-3 years, and what could that evolution suggest for how "Web 2.0" might unfold?