After the switch to OS X Lion, many users discovered that Apple had flipped a few switches that made the OS feel… different. Some of these changes made the system more user friendly, but others are just plain awkward. To restore the system back to its former self, check out this list of commands on Github. Some examples include disabling Resume for Preview.app, automatically opening a new Finder window when new volumes are mounted, disabling autocorrect, enabling key repeat, Dashboard developer mode (which allows widgets to stay on the desktop) and many others.
For more hidden settings, also take a look at Secrets, a database dedicated to such tweaks.