O'Reilly have a nice article on "What is Web 2.0 (What is Web 2.0)":http://www.oreillynet.com/pub/a/oreilly/tim/news/2005/09/30/what-is-web-20.html and Jason point out "The top 10 things that aren't Web 2.0 (The top 10 things that aren't Web 2.0)":http://37signals.com/svn/archives2/the_top_10_things_that_arent_web_20.php.

PJ Heyett seems to know "exactly what web 2.0 is (exactly what web 2.0 is)":http://pjhyett.com/articles/2005/10/03/priceless-definition-of-web-2-0! He says it is

* pretentious
* oversized fonts
* pastel colors
* buzzwords
* featureless ǃ?user experiencesǃ?
* overly friendly and self-important copy
* acronyms

__Update__

Here is another funny "Web 2.0 Checklist (Web 2.0 Checklist)":http://msippey.tadalist.com/lists/public/155420

It bullets as

* Give us your email address, we'll let you know when it's ready!
* Public -beta- alpha
* Tags
* Feeds for everything
* Built with Rails
* Sprinkled with Ajax
* Yellow fade
* Blue gradients
* Big icons
* Big fonts
* Big input boxes
* REST API
* Google Maps mashup
* Share with a friend
* TypePad blog for a peek inside the team
* Feature screencasts __(thanks, Waxy!)__
* Hackathons for new features
* Development wiki
* Business model optimized for the long tail
* It's Free!/AdSense revenue stream

Reference Links

* "Web 2.0 Bingo (Web 2.0 Bingo)":http://web2.0bingo.com/