1. When opening the game, don't mess with any other setting and go straight to the brightness slider and try to set it right or left. Brightness does not change at all.
2. If you change for example the hardware antialiasing to any value and then try again changing the brightness slider now works properly. Works changing hardware antialiasing but might work messing with other settings too. If you want to reproduce the third step change the slider to something brighter or darker than default.
3. Having changed the default brightness, exit the game and then start DayZ again and even that the brightness slider is in the position that you have left it changed, brightness is the default one. Moving the slider does not change brightness so again we are in step one.

I have tried this in three diferent computers with the exactly same steps and results. This is important because in one of these three computers the screen is kind of darker than the others and I really need to increase brightness just a bit to be able to play since the auto exposure feature makes it so dark in the forest. It is anoying getting this slider to work everytime I start the game. I can change my overall display brightness but unless playing Dayz in forest, I don't really need more brightness in my OS. This slider should really work.