If you installed Apple’s latest beta of OS X, El Capitan, you may have noticed the LCD appears to be a bit brighter with less contrast. I didn’t notice a problem until I launched Diablo III and found that it was almost unplayable due to the screen brightness.
After tinkering, I realized that El Capitan has added an option in the Display preferences that allows your Mac to automatically adjust the display based on ambient light. Turning this option off (unchecking) fixed my screen brightness issue.
Because this is an iMac, in an office where I control the lighting, this is probably optimal. However, I haven’t installed the beta on my MacBook Pro, so I’m not sure if the new feature offers benefits to mobile users.