web 2.0 isn't a function of design, it's more about the nature of the interactivity that the site offers. my favorite web 2.0 site is pownce.com because it's made by Leah Culver, Kevin Rose and Daniel Burka.....3 VERY SEXY individuals. And it comes through in the site, they are sexy sexy people!
I think web 2.0 has a little to do with design as much as it does the nature of interactivity... maybe a collection of things perhaps? Oh and my favorite web 2.0'ish site is live.com