What's New in Mac OS X 10.3 Panther
Disclaimer: although I am an Apple Certified Trainer by day, this guide is not endorsed by or affiliated with Apple in any way, nor is it indicative of future Apple Training courses. It is based entirely on my own personal experience and research. Any inaccuracies are my fault. Comments and corrections welcome at firstname.lastname@example.org.
Miscellaneous System Preferences improvements
The Desktop pane and Screen Effects pane of System Preferences have merged into one, called Desktop & Screen Saver. In addition, you can now select a desktop picture directly from your iPhoto library.
Exposé is a new feature in Mac OS X 10.3 that allows you to temporarily shuffle all your open windows to see them all at once, then clck on one to bring it to the front. You can configure keyboard shortcuts for this in the Exposé pane of System Preferences.
Speaking of keyboard shortcuts, the Keyboard Shortcuts tab of Keyboard & Mouse is greatly expanded. You can customize shortcuts for a laundry list of Finder and system-wide functions, including screen capture, Universal Access settings, full keyboard navigation, and auto-hiding the dock. There is an (empty on my fresh install) place at the end of the list labeled "Application shortcuts"; you can click the little "+" button at the bottom of the list to add an application, and then customize its shortcuts. This works in most applications, although you may need to quit and relaunch the application for the changes to take effect. (Thanks Quinn for explaining this feature to me.)
Software Update now has an option to auto-download critical updates. The feature is off by default, and even when it is turned on, the updates are only downloaded, never installed without your approving them.