Just a nicer Ribbon UI applied in all applications including Project and Visio in 2010, new icons (again), and tight SharePoint integration.
I like them all, except SharePoint Designer (though it's now a free download) - you have to have SharePoint Foundation Server 2010 installed (on the same computer or on a network - mine is on a network) in order to use SPD.
New 64-bit edition alongside 32-bit - hallelujah!
Groove 2007 has been renamed to SharePoint Workspace 2010 - dumb move IMO.
InfoPath 2010 has been split in 2 - Forms Designer and Forms Filler - in my opinion, that's a dumb move.
Office Accounting has been discontinued (last version is 2009) - hallelujah.
Visio has a new edition added - Standard ($300-something) and Pro ($569) is the same as before, and a new one "Premium" - advanced diagramming and stuff. More expensive than the Pro - I saw that it costs a thousand bucks retail.
The naming scheme (like "Microsoft Office Word 2007," etc) has returned to just "Microsoft Word 2010," etc BUT is still in the Program Files folder "Microsoft Office." While I can't speak for the customers who wanted name simplicity, I can understand complexity just as well. So, I renamed all of them to "Microsoft Office Word 2010," etc.
All in all though, 2010 is a nice upgrade to 2007 - like when 2003 is a nicer upgrade to 2002. I'd recommend 2010 to all.