I figured that Yahoo would buy del.icio.us, actually.  With flickr, and Yahoo's commitment to tagging, seemed the perfect match.

This is a good article about Web 2.0. 

A couple of things -

a. I would add to this, that the web 2.0 applications, are, for the most part hosted, and therefore always improving in real-time.  Google Reader is an example - while it's first iteration was blase, it quickly added the necessary features, and continues to get better, as do all the other hosted applications.  Especially given the "lightness" of most of the applications in question, it's not such a big deal to add feature sets.

Also, as discussed here, there is a challenge using Web 2.0 for "softer" applications.  Either those in HR/CRM/ERP, where complicated software is required, or, in those systems where a history and profile has to be maintained over a significant amount of time.