Developer, Former MVP, now at Microsoft - Best of 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013
I have been using Office 2010 since Beta1 and have not looked back since. I am currently on an internal RC, but will upgrade tomorrow to the RTM version.
There are a plethora of new productivity features and for Word 2010 the one that overshadows everything else, IMO, is the Navigation Pane. I could spend time describing it here, but I'll never be able to cover it more thoroughly than what the product team has on their blog post.
You enable it via the "Navigation Pane" checkbox in the "Show" group of the "View" tab on the Word ribbon.
Even if you have come across this new Word 2010 feature, trust me you will learn something more about it, you will thank me later. Go learn how to make the most of the new Navigation Pane.
As an aside, there are many new benefits in PowerPoint 2010 too, my favorite being support for sections. Not to leave Excel 2010 out, you should check Excel's integration with HPC Server.