Jensen Harris: An Office User Interface Blog : New Rectangles to the Rescue? (Why the UI, Part 4): "The Task Pane was the last attempt to find a way to scale old-style UI to programs as full-featured as Office. Although it was a successful stop-gap measure, it ran its course in only two versions. I'm reminded of Nathan Myhrvold's First Law of Software: 'Software is a gas.' Every time we add a new UI mechanism, it fills up. Because we only added and never renovated/reorganized/removed, complexity went up each release.
Office 12 is our chance to build a new interaction foundation for the next decade of productivity software."
More refreshingly frank commentary on why Microsoft is making major changes in Office 12.