Sorry, I was trying to be cute.
I would say that library 2.0 is the most important thing to understand and at the same time it is a term that is almost impossible to define in a detailed way. This Talis document goes into the detail. To me, library 2.0 is the effort to take the next step in further evolving library services to meet user needs utilizing a range of networked technologies. There are tons of people in this ning network who will have pages to offer on this, but I think that this quote may put it best:
For 2,500 years we've been told that knowing is our species' destiny and its calling. Now we can see for ourselves that knowledge isn't in our heads: It is between us.
David Weinberger. Everything is Miscellaneous. (p. 146)
Libraries can no longer be about storing and providing access to knowledge. It's about creating situations that bring people together create knowledge in an active way.