Reading through Brandon Schauer's article on the crucial DNA of Web 2.0 I was struck by an apparent oversight: if the network effect is a fundamental attribute of a Web 2.0 site/application, why are other core Internet attributes not also in the list.
For example, to my mind, a fundamental attribute for a successful Web 2.0 application is the asynchronous nature of the user's interactions with other users and with the application itself. That is, the application gains value from the fact that one user's interactions can take place days, weeks or months apart, and the value of that interaction is retained.
Another example for me is that of 'infinite virtual capacity' [Afuah & Tucci, 2001]. Web 2.0 brainchilds such as flickr would be far less valuable if not for the transparent manner in which the site's operators can continually expand the capacity of the system. If, instead, users were constantly confronted with limitations and allotments, the take-up - and therefore the value - would be considerably lowered.
[Afuah, A. & Tucci, C "Internet Business models & strategies" McGraw Hill, 2001]