To most, this question might feel subjective... but there's really more than just opinion involved here.
If you hover your hand over something in real life, most times a subtle shadow will appear above the object. Mimicking that same idea in the form of user-interface / experience could be more intuitive or natural... or not if the user perceives technology as something totally separate from real life.
An example of the two: http://jsfiddle.net/ganoucgk/
So again, what is more intuitive to the user, a button that is illuminated upon hover, or darkened upon hover? And why?