Well if you're going with physically based stuff a filmic tonemapper: http://mynameismjp.wordpress.com/2010/04/30/a-closer-look-at-tone-mapping/
They're all, supposedly if the name means anything, based off the tonemapping for actual film, which deals obviously with physically base "real world" stuff anyway. There was this huge spiel with data, a pdf, I read months ago explaining exactly why film exposes in that way and how it suits real world scenarios, including real world under/over exposure clipping and light ranges and etc. If anyone knows what I'm babbling about and has it bookmarked then that's the most helpful thing I can think of. Real world light intensity of everything from a moonless night to the middle of a bright day and etc.
One of the specific I can remember was a reason for the toes on either end spreading out in an S shape, and that was to avoid sharp clipping in either over or under exposure, as well as ensuring there's still some saturation in both relatively dark shadows and bright highlights.