How can I bring back the "bounce" when scrolling in OS X?
I think you may be referring to the "Scroll with inertia" option, which is found in System Preferences > Trackpad. If it's set to scroll with inertia, but inertial scrolling isn't working, try toggling it to "without inertia" then back to "with inertia".

If that doesn't work, you can try deleting the preference file, "com.apple.driver.AppleBluetoothMultitouch.trackpad.plist" found in ~/Library/Preferences. Then log out, log back in and check the Trackpad preferences again.