An interesting bit from a summary from a Keynote by Martin Fowler:
Requirements gathering is also very tough on large projects. Martin provided an interesting comparison between writing requirements documentation and writing books. Writing a book requires a very large investment of time and energy. Even after peer reviews and feedback from professional editors, the author still is not always successful at communicating his/her intended message to the audience. His point was that if authors have a hard time doing this with books, imagine what it must be like to capture the requirements around a complicated business application given far fewer resources.
If a professional author is not always capable of communicating his/her intended message, what chance does the business have in getting their requirements across to IT with far less time, expertise and resources.