I'm using Opensuse 13.1 with KDE 4.11.5 and have a dual monitor setup with a Nvidia geforce gtx 650ti card (latest drivers).

The problem is when I run unreal tournament via wine with openGL renderer (fuslcreen or not), the in-game brightness is applied only when I move the slider for split second, then it goes to default.

I've tried to reproduce the problem under ICEWM and TWM, but it works fine. It even resets the brightness if I replace kwin with twm - some component inside kde keeps doing that.
Any clues what that could be?

Found the problem. I've forgot about f.lux I had installed a long time ago - it is set to start with KDE And seems it causes the issue.

Thanks for posting the solution.