> My theory is that the set of aesthetically pleasing images which look good on
> properly calibrated monitors will all have a similar histogram.

It surely depends on the content of the image (skiing scene, sunny 
beach, overcast fields, moon at night)?  Did you pick a few random 
images (eg from google or flickr) that look good and compare the 
histograms?  My expectation is that they would be quite different.

> I would guess that when you write an algorithm for a digital camera to chose
> exposure time, you'd just leave the shutter open long enough to hit some target
> RGB value.

Indeed, usually the camera tries to get the average pixel value to 
something like 17%.  On more expensive cameras you can select the region 
over which this average is done (entire frame, center weighted, spot, 
etc).  This works well enough most of the time, but a serious 
photographer will use manual exposure and look at the histogram.  A good 
example is snow, auto exposure usually gives pictures that look way too 
dark (I assume because the camera's "17% rule" is assuming the sky to be 
way brighter than the ground, in snow it often isn't).

Usually you can't just take the range of pixel brightnesses and map this 
to 0-255, a small number of very dark and very bright pixels will cause 
most of the image to lie between 100 and 140 or something and look very 
washed out overall.

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