I'm teaching currently from a book "Content Networking" and I notice that the authors classify all the following under the topic "P2P Systems":
* File Sharing (Napster, Gnutella, Chord, etc)
* Collaboration (Groove, Magi, AIM, etc)
* Distributed Computing (SETI@home, Gnome@home, etc)
* Platforms (JXTA, .NET, etc)
I contend that it's more that these things came to prominence at the same time (i.e. zeitgeist) rather than some well-defined, technical commonality forming this group of terms.
Similarly, I look at the big bunch of stuff that Tim agglomerates into the topic "Web 2.0" and see more zeitgeist than sharp concept delineation. So maybe Web 2.0 is just Web 2005? If so, then I look forward to Web 188.8.131.52.