Overall, the best and clearest definition of Library 2.0 that I found was located here. This article greatly raises the necessity of using the successes of certain user-centered Web technologies as a way to restructure libraries. Not only did it raise the possibility of a radical paradigm change, it also brought simple ideas to the table, such as renting ipods to people.
Among the several definitions tossed about, I think that the clearest would be that the concept Library 2.0 denotes a restructuring of libraries to take full advantage of current technologies and people's ideas. Using OPAC in ways similar to Amazon, where people can leave reviews, rate books, or constuct recommendations for others would be a prime example. Such a user-centered system would allow people to wade through the library catalog with a better idea of what they are looking at without forcing librarians to construct every tedious detail. It would also give librarians a good indication of what is truly popular without necessarily having to look at raw data, such as number of total checkouts of a particular item. Such two-way communication between users and librarians would facilitate better service. Most likely, such a system could also be adapted to provide better feedback on library events and classes.