This list is NOT comprehensive or complete.  Its just the changes I have observed or find interesting.

• Change search locations.  Change the default behavior of Spotlight to have it search the currently selected folder or your most recent search location. Go to Finder -> Preferences -> Advanced -> change “When performing a search:” to “Search the current folder”.
• Spell checker in TextEdit.  Can be controlled in TextEdit preferences.
• Airport menu now shows signal strength of available wireless networks.
• Minimize apps onto their doc icon, instead of the right side of the doc. Go to Apple Menu -> System Preferences -> Dock -> select “Minimize windows into application icon”.
• Activate Expose from the dock:  Click and hold an application icon in the Dock and all the windows for that application will unshuffle so you can quickly change to another one.
• Expose shortcuts:  With all windows visible in Expose, you can press Command-1 to arrange the windows by name or Command-2 to arrange them by application. Pressing the Tab key will cycle through all the running applications. Press the spacebar while pointing at a window to expand it.
• Expose windows are now highlighted with a blue outline when mouse over.
• More reliable disk eject.
• Restore deleted items to original folders.
• Scroll though stacks and navigate folders in stacks.
• Faster shutdown and wake-up.
• HFS+ read support in Boot Camp.
• Screen recording functionality built into Quicktime Player.  Seems to only support the main screen, not external monitors.
• Date in menu bar.d
• Screen saver shuffle.
• Multi-Touch gestures in older Mac models.
• Preview intelligent text selection.
• Location awareness:  Snow Leopard can determine your time zone from information provided by your Wi-Fi hotspot – in your Date & Time preferences, choose Time Zone, and tick Set Time Zone Automatically
• Descriptive Screenshot filenames: Instead of “Picture 1,” you now get “Screen shot 2009-09-04 at 11.26.49 AM”  (It seems they didn’t follow this same goodness with their screencast recordings.  Always hope for their next patch?)

Here is what broke on my upgrade:

• Little Snitch
• iStat Menus