In Fedora 21 Beta, trying to adjust the screen brightness using the top-right menu's brightness slider has long, up to 10 second delays in actually changing the hardware brightness. This causes the actual screen brightness to play "catch up" with how I moved the slider trying to get it to work, wildly changing settings until it lands on that which corresponds with where I left the slider when I gave up.
That sounds like a problem with the settings application for GNOME, rather than something pertaining to the Wayland protocol. I suspect chances are good that in the intervening time since this was reported, the issue has already been resolved. But if not, please re-file the bug with Fedora rather than here.
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