Web 1.0 was about building websites to publish contents in HTML and sell things. Search engines are the single point of access for most web users. The higher the hit rate a website has, the higher the value it has.
Web 2.0 is about using the Web collaboratively — sharing and mixing up information and resources. The whole content pool is being analyzed by millions of talented web users. Users can consume or create contents in a personalized way either through blogs or wiki.
Search engines provide ad-hoc searching for unorganized contents as well as channels to deliver targeted contents. The playing field is spending money inside the Web to promote websites by creating as much links as possible. The higher the number of links a website has, the higher the value it can generate.
O’Reilly gives as examples: eBay, craigslist, Wikipedia, del.icio.us, Skype and Adsense