If you're migrating from 2007, then you wont find many changes. But I can summarise major changes as
Excel 2010 has about 50 new formulas, mostly related to statistics.
Sparklines are sexy stuff, esp if you believe in the power of graphics and images or simply are an Edward Tufte Fan.
PowerPivot is an amazing Addin ONLY FOR EXCEL 2010. This can handle massive amounts of data. I saw one video handling about 18 million rows of data. This is a Business Intelligence Tool, and is an awesome addin
PP '10 has better effects, they improve on the 2007 base. There's a sample Presentation by Nancy Duarte in the PP 2010 samples, check that out, if you've installed 2010. It's an awesome presentation created solely in Powerpoint. Also, effects are more "unhidden"
PP now has video editing capability. I've heard really awesome reviews about that, though haven't used it myself. But inserting and playing videos is easier in 10 than '07
You can export a Powerpoint presentation to a .wmv video
Performance is much better and you can minimize the splash screen(useful when loading super large excel sheets)
The Office Orb(File Menu) has been replaced by a file tab known as the backstage view. I love the option to save to skydrive and PDF printing capability is given by default.
The tabs are seamlessly merged with the Title Bar. Themes are slightly different and are more sober looking. Better effects of the Title Bar under Aero for Win7/Vista.