Targeting casual use was the key. Even in the "professional" world almost nobody uses the Office applications to their full extent. The people who can are using more appropriate software. Accountants have Intuit and Peachtree, Anyone working in graphics, design or typesetting will use the appropriate Adobe app or competitor. Power Point is probably the only member of the suite that doesn't have a more "professional" equivalent. But there are definitely people who are more adept with it in the business world.

Office was probably feature complete in its second or third iteration. The only substantive changes 99% of users notice are in interface overhauls.