The first thing I noticed when I moved from Office 2003 to Office 2007 was the vastly changed menus in Word. There is also no way to change to the classic menu view that Office 2003 users are familiar with. Heavy users of Office will certainly see a learning curve with the new menus.
Addintools has a product called Classic Menu now available for Office 2007 that puts Office 2003 style menus back in Office 207 without sacrificing the new Office 2007 features.
Addintools has a free trial of the Classic Menu software available so you can see if it improves your workflow. If you like it, the full version of the software is $29.95.