Active Web - formerly was Web 2.0
From my viewpoint, there is a big commonality among all those concepts and sites. Everything in Web 2.0 is about (inter)activity.
In AJAX and similar technologies - the web page is not passive anymore. Instead, it contains code fragments that actively pull/push information from/to backend servers to achieve a better user experience.
In social networks users are not passive consumers anymore, but actively connect with each other. To enable this, web sites must actively connect different people with each other.
In pages such as Flickr, YouTube, Digg, Amazon people actively share information and media. Podcasts and VideoPodcasts allow people to become active content providers. Same for P2P networks, Wikis, Blogs, ....
In other words, Web 2.0 should be better renamed to reflect the aforementioned observations. It is about the evolution of the Web from a passive medium where we find a small group of active content providers and a large group of passive content consumers to a new Web where the boundaries between content providers and content consumers more or less disappear. This new Web provides technologies and means for Web users to switch from a passive consumer to an active provider almost immediately. It is a Web where everyone can participate. It is no longer a technology platform dedicated to large companies for marketing purposes or establishing Web-shops. It is more about social interactions and active participation. It is an open medium for everyone.
My suggestion thus is: let us rename Web 2.0 to Active Web. I think, this name much better reflects what the new Web evolution is all about.