Quickly hiding the Dock!
Having the Finder automatically hide and show the Dock is really too much of a slowdown for me. Depending in which applications I am working in, however, I sometimes want to have it temporarily hidden. It is easy to do so with a quick keyboard shortcut. The shortcut "Option - Command - D" will turn hiding on and off. The same can also be achieved by right-clicking on the Dock, between the applications icons and the stacks, and choosing "Turn Hiding On". It is much easier than going to System Preferences every time you want to change this.