Hmm...I guess I'll have to go in depth on this subject in my report when I finally get around to writing it (I have two study halls every day in school starting tomorrow, so lots of free time!)
I find it interesting to note that the ancient pagan gods (of any pantheon it seems) had to deal with pretty much day-to-day matters just as if they were humans, as well as humans looking to them for help. So the development of the gods would depend largely on everyday things, climate and weather patterns specifically, but also some other things. This would cause the beliefs to vary, possibly even from one part of Rome or Greece to the other (of course once Rome was pretty much unified this would likely change however individual "parts" would still retain some individuality of religion. Greece, since it was seperated into city-states and not really unified, would remain much more individual). Celtic gods have a distinctly "Celtic" feel about them, Roman have a Roman "feel" and characteristics, etc. Also, different sects of society had slightly differing religions (this is where the "cults" come in, such as the Cult of Mithras)