The term Web 2.0 has evolved to have two meanings depending on who you speak to. What started off purely as a structural/functional term to describe the function of a website has also evolved in to a design term to also describe the style of a website.
This is mainly due to the websites that were/are being championed as great examples of the Web 2.0 movement shared a similar visual style. By working to web accessability guidelines and using techniques such as gradient fills and rounded corners designer could create a simple design that was easy to use and still had a lot of impact. Be cautious though as, simple as it may look, it is hard to do well.