I very much appreciate a number of the new features, particluarly live preview and a lot of the new formatting options available in such tools as Excel.
A lot of people won't really find the OpenXML format to be useful (.docx, .xlsx, and .pptx), but that's pretty cool too. For those of you who don't know, the Office 2007 file formats are series of XML files and resources stored in a .zip archive with the appropriate application extension.
I also appreciate that in Word and Excel, at least, they've gone to lengths to keep all of the menu shortcuts available so that I could use the keyboard shortcuts I've come to know and love.