File format change - annoying, User interface change - extremely annoying
What I most object to about Office 2007 is the fact that, if I want to generate documents in the new format, I have to use Office 2007 which means having to use the "Ribbon interface" which is a dreadfully inefficient system compared with the drop-down menu system in Office 2003 and earlier versions. MS made Office 2007 capable of creating files in the earlier formats but they didn't allow users to opt for the older user interface which, as I have to work with the wretched thing every day, is a MAJOR omission.
On the subject of backward compatibility, storage is so cheap these days that Office 2007 should have produced documents in BOTH file formats so that users of Office 2007 could deal with the new format and any advantages it may have while Office 2003 and previous users could still read the old format version.