This must,by necessity, ban most books. There simply isn't shelf space. The result will always reflect the personal view of the deciding body.
Re: banned books. Some are banned, some are "challenged" meaning someone tried to have them banned. The list is long and some of the reasons for banning a book may seem silly. Some of the books on the list in this latest attempt at creating a Palin scandal were banned a long time ago or in far away countries.
Re: librarians as censors. While it's true that there is a limit to shelving space in any library, the librarians who do the purchasing select according to the interests/needs/demands of the population they serve. If the book you seek is not in the collection of your library (whether it's an academic library or a very small public library), the librarian will get it for you through inter-library loan or, at the very least, let you know where it can be found. A librarian does not censor information, they make it accessible.