I only skimmed the responses, so I don't know if this was mentioned (I know many other good things were):

Depth of field is important, and you should put it at the top of your list.

A camera generally shows us sharp focus on objects in a relatively narrow slice of space (depending primarily on apperture and lens factors).

This generally means far background objects and close foreground objects should be out-of-focus to some degree.   Our eyes and our subconscious immediately pick up on a scene where there is supposed to be depth, yet everything is in sharp focus as "fake".  This kills the illusion of realism.

You can do this inside the render (warning -- increases render time a great deal) or you can chaet it after the render by adding selective blurring to objects yourself.

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