One other thing that may have added to the developement of Greek philosophy--their government was not based on a religious instituion. In Mesopotamia, there were various temples whose priests legitimized the various rulers. The very idea of kingship was said to have come from the gods, and humans were said to have been created in order to do the heavy work the lesser gods got tired of. In Eygpt, the pharoahs were considered divine, and their earthly existence was tied up in endless ceremonies. Needless to say, they were even more intimately tied to the divine after death through mortuary temples and such.

All three of these societies were polytheistic, though there was a brief expiment with monotheism in Egypt. Classical Greece had perhaps the most firmly established and hierarchical pantheon (Zeus being the father of the gods, etc.), while Eygpt and Mesopotamia has more amorphous pantheons.

Regardless how they viewed their gods, it was the how the religious institutions excercied authority on earth that led to either freedom of thought or the absence of it.

I hardly think this is the final word on the matter. I'm just hoping to add another diminsion to the discussion.