Many of the Office 12 UI decisions were influences by what was learned in collaboration with the usability and research team. Jensen Harris explains that it is more than just the two-way mirror.
Jensen Harris published on the subject before (Be willing to be wrong) on the design of “the Ribbon” and got lots of responses worth reading.
From the list of all his Office 12 UI posts, Tipping the scale gives a more visual look at the scale issues facing Office. Here are a few charts, demonstrating the number of top-level menu items, toolbars, and Task Panes included in the product, from Word 1.0 to Word 2003: