The biggest change for experienced users is the introduction of ribbons rather than menu bars. Once you accept that fact you'll find that you can actually find features easier than before. In fact, you may assume that many features you find are new to PowerPoint 2007, where in fact, they probably existed before buried under a multitude of menu options.
For example, the following illustrates the choices available on the Slide Show ribbon. Most of these features are not new but can now be easily accessed from this ribbon.
All ribbon articles in HelpDesk Online are clickable (even this one). Just click on a button to learn more about that feature.
Click on the Microsoft Office button at the top left corner to access the old File menu choices and so much more.
There are very few toolbars in PowerPoint 2007. You can, however, add to your Quick Access bar . (It's beside the Microsoft Office button unless you move it.)
If you're more than just a basic-features user, you'll probably want to take a look at the following articles sooner than later: