Since Revit 2010 has been formally released and began shipping over the last couple days it makes sense to begin addressing it formally here too. I've held off doing so to honor the disclosure requirements I agreed to with Autodesk even though numerous others have managed to skirt them for sometime now.
The first thing that will "hit you" obviously is the new graphical user interface (GUI). You'll want to visit the Help documentation and there are several items of interest there.
Skipping over the first item, Help, check out the "User Interface Overview".
It is a "mouse over" document where you move your cursor over the numbered portions of the interface and a description appears. It lacks descriptions for a couple contextual items that appear from time to time. For instance there is a Draw/Pick Gallery that appears when you are working with a feature that permits sketching, like walls, lines etc.
Subtle yes but it is useful to learn their terminology too.
I also highlighted a document you'll definitely want to get familiar with too. It is called, not surprisingly, "Where is my Command?".
This is organized around the original menu structure and provides a description that explains where this "missing" command can now be found. Here's an example that will cause a little stress initially unless you are already quite indoctrinated by other "wibbonized" applications. Where did they hide Settings menu > Options? Sorry, that was a trick question, they don't tell you where it is!! It is found here.
Initially I expected to find it on the Manage Ribbon tab however I soon remembered that in other applications the Options or User Settings are configured under the Application menu. This is the BIG round button in the upper left corner of the application. In Revit's case, the BIG "R".
If you are interested in some background on the rationale for the new interface design you can check out the new Autodesk blog "Inside the Factory". There are also quite a few videos posted on youtube and other sites. Some critical of it and others just a demonstration. It's amazing how much stuff "we" create on the "internets".
Just be sure to check out the available information lurking under Help!