The Office 2007 ribbon
I'm still a bit uncomfortable with the new Office 2007 ribbon. IMO, maybe it will just take time, but it's a very difficult twist to use the new ribbon feature. For anyone who has created custom toolbars for apps, your buttons now fall into the Add-ins ribbon. Also, the user doesn't have the option of reorganizing the ribbon. Granted, this makes a very consisten interface, but there are many more options in Office programs than are displayed. I get irritated with the quick access toolbar as my only other option.
Anyways, from the developing perspective, the new interface is a huge improvement over the old CommandBars interface. Here's a good Office Beta 2 link on some development around the IRibbonControl interface.