There are some features in Snow Leopard that Apple try's to hide from it's users. I don't know why but you can't find those Settings in the "System Preferences" now anywhere else.
Here are my favorites:

1. Animated Preview for Folders in QuickView

defaults write com.apple.finder QLEnableXRayFolders 1

[youtube=http://www.youtube.com/watch?v=145XIc5pFEg]

2. iTunes green zoom button
Since iTunes 9 you get the MiniPlayer when pressing the green button in itunes. To get back the max. function you'll need enter this:

defaults write com.apple.iTunes zoom-to-window -bool YES

Then restart iTunes and be happy.

3. Re-enable auto play for QuickTime
The new Quicktime does not have any preferences, because of this you can't enable AutoPlay by default. Here's the command to enable it:

defaults write com.apple.QuickTimePlayerX MGPlayMovieOnOpen 1

4. Keep QuickTime in fullscreen when switching applications
If you are used to work on multi screen systems, sometimes you wan't to watch a movie or sth. else on another screen. With Quicktime this can be annoying. If you switch to fullscreen and then use Command-Tab to switch the applications QuickTime will automatically quit fullscreen. Type this to get rid of it:

defaults write com.apple.QuickTimePlayerX MGFullScreenExitOnAppSwitch 0

5. screensaver password delay
New SL feature is to set a time delay for password requests. How long does the screensaver had to be active before it requires a password. Do it with:

defaults -currentHost write com.apple.screensaver askForPasswordDelay -int 900

900 == 15min

6. Disable Dock Expose
When clicking on a dock Icon in Leopard you got the "right-click" contex menu. In SL it changed to the DockExpose which will show all current windows of the app. To disable this type:

defaults write com.apple.dock show-expose-menus -boolean no; killall Dock

If you are looking for other hidden settings the best ressource to do this is http://secrets.blacktree.com/.