SharePoint Integration, InfoPath
Glossed over in a previous post but the integration with SharePoint/WSS in MSO 2007/2010 is not to be sneezed at. From a productivity point of view alone it's very useful, speeds up colaborative work tremendously and saves huge amounts of cash when developing apps within the enterprise.
Very open to O.O and simmilar where they fit not just tying to blow an MS BS trumpet BTW. I'm pretty much platform agnostic but living in the current state of play not the imagined one.
To my mild suprise my favourite addition to Office 2007 was OneNote (No really... ) I don't know why especially. I liked planning larger projects out in notebooks with tabs post-it notes and various addendums and OneNote feels a bit simmilar but searchable I guess? I thought it added value to the overall offering anyway.
Also no one mentioned InfoPath 2007 which is an excellent and useful product within any appropriate business by any standard.