I suspect the brightness problem we are seeing has to do with some post-processing filter (perhaps a bloom filter?) that might require some tweaking. I seem to remember that at some point there was some kind of eye-adaptation simulation available but didn't work so well. In fact that might be the key to it as things are supposed to be blinding when coming out from a dark environment and the opposite effect should occur when entering into one. An implementation would temporarily brighten all pixels in frame and apply the bloom filter on top of that when coming from a dark environment into a bright one. As the eye adapts over a few seconds the pixel-brightening filter would progressively reduce its effect and the threshold for the bloom filter would increase, so that only looking into bright light sources, i.e. the sun, would burn nearby pixels and even blind the whole frame.
The concepts and even the algorithms are not particularly complex, but I suspect it is non trivial to have assets in the right value-range, i.e. light sources, texture values and filter factors. I.e. the sun might have intensity 100.0 or even 1000.0, while a torch might be a mere 10.0 and the pixels in a glowing rune texture might be a dim 0.5. In daylight both rune and torch would not really have any visible effect as the sun would swamp their output. As night falls and the eye adapts however, a torch might become quite a bright source and the rune would start glowing feebly. But wrong filter factors might turn perfectly reasonable light intensity values in unacceptable results.
Still, if the right interplay between assets and filter is found, the results are spectacular. And it also introduces the potential for some interesting gameplay, i.e. luring a player in a dark cave and taking advantage of its temporary incapacity to see through the darkness to attack him. Or blinding everybody nearby with an otherwise harmless flash spell. I suspect begla might have set it all up already. Perhaps we just need to find how to make it all work.
Easy fix is easy! Can always make it a setting you can configure. 
