Web 2.0 has a few meanings: 2nd generation web, business model by advertising, AJAX. Or you can view the WIKIPEDIA definition, but to me it represents more of a shifting of the business paradyme. What does that mean?
Well I use to think that Google was in the Internet business, but after a few discussions and conferences I realized that Google is actually in the Media business, selling advertising, owning information, search, video, news, etc.
I also use to think that building a website was about selling software, but after trying to launch a few sites and having them not really take off, I realized this is more about the marketing business, and selling eyeballs.
Of all the other companies out there getting buzz, like DIGG, GoBigNetwork, Technorati, etc. They are not in the software business, their business models are something else.
So in short, Web 2.0 is the re-birth of the web as selling something besides software, it's selling some other product or service people need, and the website is just the way you distribute this product or service.
Want some advice on starting a Web 2.0 company? Don't model your company as a software company, create a business that sells a product or service, know your custom that wants to buy your product or service VERY, VERY WELL, then build a site around this product or service in such a way that the delivery is unique, and cool, and most important remember to market this site with everything you have!