To my mind, the difference between the two depends entirely on what you consider the term "god" to be a valid descriptor of. As far as I'm concerned, all religions are either polytheistic outright, or employ polytheism in significant volumes of their denominational subsets. Judaism and Christianity have angels and demons. Catholicism has saints. Islam has prophets. Before these belief systems gained force, similar beings in older religions would have simply been called minor gods.
So, really, the difference between the two is not a matter of what their respective adherents believe at all. The difference is in what the systems value and encourage. And it's clear that, all things being equal, polytheism encourages individualism, whereas monotheism encourages purity. They both encourage unity, but in different spheres; monotheism wants a culture to be united by all of its members sharing the same beliefs, while polytheism wants a culture to be united by more of a "scratch my back and I'll scratch yours" attitude in its members. Again, I want to point out this is all in terms of "pure concept" - the reality, of course, is that all religions employ some mixture of these elements.
But the examples do seem to support this. Christianity, for example -Catholicism in particular- has an especially strong emphasis on the concept of purification; from the Eucharist, to baptism, to penance for more severe trespasses, to some of the torture methods of the Spanish Inquisition, there is an incredibly strong streak of a desire to root out the inner cancer that blights the soul of the pagan and the heretic. Combine this overarching trend of a will towards "true essence" with more specific occurrences like the so-called "cadaver tombs," the architecture of Romanesque and Gothic churches as a way to make the human lose sight of himself via his immersion into something much bigger, and its view of "evil" as an active, vibrant, eternal force, and it becomes no wonder that death and black metal have shown such an infatuation for the whole phenomenon. Its development into a sort of pan-European philosophical system that combined the refinement and expressiveness of southern European cultures with the wanderlust and vir of the northern Eueopean ones probably also plays a part in this, albeit subconsciously for the most part.