Yes, this is true - but also many social justice movements have been put forward by monotheists. I'm not saying this as a sweeping statement that it has always been a universal force for good, nothing ever has or ever can be, but that it has by and large been a movement for social justice - whether that social justice is your brand or otherwise.The major religions have certainly been a force for social justice in particular contexts, as they have also been a force for oppression and violence in other contexts. I think it's extremely difficult to try to draw some overarching theme of what the consequences of all monotheism (or any religion of sufficient size) has been.
Yes, but the difference is in the inherent human worth.I would say that is an extremely inaccurate, or incomplete, portrait of the history involved. Monotheism did not invent the notion of all people being equals - my understanding is that the animism of many indigenous cultures, for example, reinforced a quite egalitarian, democratic ethic in many places. Monotheism, on the other hand, has frequently reinforced rigid hierarchy (e.g. between the priestly and lay classes of society, between sexes, etc.). The divine right of kings, for example, was a concept that enabled monotheistic rulers to claim de facto divine status and the authority to speak on behalf of the One True God. We see this concept all the way back in the Torah, which elevated Moses' status to the literal spokesperson for Yahweh. That power dynamic is extremely ripe for exploitation and has produced much oppression.
In polytheistic societies one's moral worth was tied up with one's social status. This is especially true in Plato's Politeia, where he actually advocates lying to people about themselves to keep everyone in their socially defined status. The man is morally worth more than the woman; he is greater in human worth and dignity by virtue of his being a man.
Monotheism, on the other hand, does not get rid of hierarchy, but what it does is makes each human morally equal in that they are all responsible to the same transcendent power. The King may have had a Divine Right (although this is a fundamentally Christian idea), he was still morally equal in his fundamental status as a human being to other human beings (whether this played out politically is another matter, but this is the theological belief).
When you say everyone is beholden to the same standard, the same transcendent power, they become more as brothers in the system where, albeit there is a hierarchy, as souls, as humans, they are the same. This was one of the main driving factors behind the rapid spread of Christianity through Greece and Rome, because the idea that 'all are made in G-d's image' and 'all are one in Jesus' was such a novel concept to the folks in those societies that it was revolutionary.