Current as of Yosemite. (Based on user comments on GitHub, probably works fine for El Capitan too.) Use at your own risk.
Expanding the save panel by default
defaults write NSGlobalDomain NSNavPanelExpandedStateForSaveMode -bool true
Save to disk, rather than iCloud, by default?
defaults write NSGlobalDomain NSDocumentSaveNewDocumentsToCloud -bool false
Disabling press-and-hold for special keys in favor of key repeat
defaults write NSGlobalDomain ApplePressAndHoldEnabled -bool false
Disable the warning when changing a file extension
defaults write com.apple.finder FXEnableExtensionChangeWarning -bool false
Allowing text selection in Quick Look/Preview in Finder by default
defaults write com.apple.finder QLEnableTextSelection -bool true