According to Diggnation, Microsoft ran a number of focus groups for Office 2007 and asked people what new features they wanted Office to do. They gave a list of needs and nice-to-haves. Funnily enough, all of the requested functionality was in Office already, but nobody could find it because it’s all hidden away in the many-layered UI.
This is an important lesson – if your users can’t find a feature, it may as well not exist and you’ve wasted your time making it. The Ribbon menu has done a lot to fix this problem, since it only shows you the features relevant to what you are doing at that particular point in time.