Please Don't Use .docx
Problem 1: They changed the interface big time. Microsoft says it's an improvement based on user data. If you, like me, value your time relearning a new menu structure that is in no way intuitive is not an improvement.
Solution: There is an easy solution - OpenOffice, the free office suite that is interoperable with Microsoft Office let's you stay with largely the same interface as MS Office 2003.
Problem 2: They changed the file format. For reasons nicely explained by Brian at Teach Them Well, this is not an improvement. It's an attempt to break the standard that Microsoft established with earlier versions of Office. The new file formats end in x (.docx, .pptx, .xlsx). These are not compatible with earlier versions of MS Office. When you instruct MS Office 2007 to use the older file formats (.doc, .ppt, .xls) the window announces that you are operating in "Compatibility Mode". Draw the inference - when you are using the new MS Office 2007 formats, you are operating in "Incompatibility Mode". Why would you want to do that?
Solution: Switch to OpenOffice or, if you must use MS Office 2007, use these instructions to change the default file formats to the standard ones.