To see when AppKit is running different code because your app linked against an older SDK (via Ken Ferry):
defaults write bundle-id NSLogUnusualAppConfig -bool YES
To slow down table view animations (via Uli Kusterer, documented in the 10.9 release notes):
defaults write bundle-id NSTableViewSlowMotion -bool YES
To show the Debug menu in Notes (via Nicolas Seriot):
defaults write com.apple.Notes showDebugMenu 1
To show the Debug menu in Calendar (via Nicolas Seriot):
defaults write com.apple.iCal CDB 1