I wanted to point out that "use cases" are different than technology implementations in Web 2.0. I've mentioned this before but I really think people need to see the difference between the two points.
An "online diary" is a use case. Lot's of people have them. Before the rise of blog implementations we called them personal home pages. The actual technology evolved over time. There are now videoblogs, photoblogs, etc. but whatever the technology, they are still online diaries.
On the work-level, a "documentation development tool" can be implemented in a great many ways. It could even be implemented in separate application tools (e.g., a forum + a wiki, a workflow app + email + content management system, etc.) There are also many variations of this documentation development tool depending upon the needs. But across all of them the use cases have some common basis.
The idea is to figure out the common/base use cases that are useful and that can be replicated on a common basis such that it can be reused by many. That's where the real challenge lies. Technology after all will always come and go.
For that same reason, I consider the Web 2.0 as a superset of all these use-cases that everyone is so interested in. It is also why "Web 2.0 != blogging", "Web 2.0 != wikis", or any one specific technology. It is the sum of all the ways we interact with the Web under the new common aspects/principles of Web 2.0 (see end of this post).
Web 2.0 use cases are different than technologies