Responsible design requires that we realize and maximize the total potential of all technologies and utilities where applicable. Responsibility not only demands we know what is available, but that we also use these technologies in an appropriate manner.
You probably won't find an exact definition of "Web 2.0" as different developers have different ideas on what the concept is. A major change, however, to the web over the years has been the appearance of websites that rely nearly entirely on the users for the content of their sites, eg "Wikipedia", "YouTube" or "Flickr". This is the notion of "Peer Contribution" and is an extremely powerful way of drawing users back to a site. Just like this one!!!