Started happening right after I upgraded from Mavericks to Yosemite.
I tried changing the brightness from the keyboard and from the System Preferences and it doesn't work either way.
The brightness indicator shows up on the screen and updates it's value when I use the keyboard, but the actual brightness doesn't change.
Also, one USB port is not working anymore.
I researched online and I found almost nothing. I did a PRAM reset, repaired disk permissions and none of them helped.