Turning off WYSIWYG font and style menus has greatly reduced my Word startup times. To do so, go to Word » Preferences (or press Command-,). In the General section, deselect WYSIWYG font and style menus -- it's third from the bottom. VoilĂ ! Faster Word. To be sure, don't do this if you like to preview fonts and styles in their respective menus.
[robg adds: Interestingly, this option is Office-wide switch -- turn off WYSIWYG menus in Word, and you'll also disable them in Excel and PowerPoint. You can also disable them in Excel or PowerPoint (the effect is still Office-wide). Of course, the setting is found in an entirely different location in those two apps: Go to Tools » Customize » Customize Toolbars/Menus, not Preferences. At the bottom right of this window, uncheck WYSIWYG font menus.]

