1. I want to post here Rebecca Lesses' comment because she is SO right to bring this up. It is the same in Christianity which is why I said in my post that whether or not you think Christianity became a monotheistic religion depends on how you view the success of the Trinity.
Rebecca Lesses: And even after the rise of rabbinic Judaism, we still find Jewish mystical texts that are very questionably monotheistic - see the treatment of Metatron in 3 Enoch, among other Hekhalot literature. (Some of the texts call him Metatron YHWH, after all). I sometimes think that it's only because of medieval rationalist philosophers like Maimonides that Judaism became truly monotheistic (and that's only if you focus on the rationalist tradition). The doctrine of the Sefirot in the medieval kabbalah is questionably monotheistic in the same way that the doctrine of the Trinity in Christianity is questionable.2. Definitions always get in the way. Monotheism is always going to be a stumbling block for us. I say along with Paula Fredrikson, RETIRE IT! for our period. Let's work out a better language to talk about what was going on. If I can pray to an angel and get help (i.e., be healed) and still be considered a Jew or a Christian, what should this be called? If I think that God can manifest himself on earth in various forms, and I start worshiping one of them (i.e., Jesus) in addition to the father, and still be considered a Jew or a Christian, what should this be called?
3. As for the issue of exclusivity. Well it does and doesn't work. There were ranges of possibilities within early Judaism. Some Jews saw Yahweh worship as exclusive - as in Yahweh is unique and other gods cannot be assimilated to him. Other Jews were fine with assimilating him with other gods. Were the Jews known for worshiping Yahweh? Absolutely. Were the Jews known for resisting his assimilation? Absolutely. But keep in mind that this was only SOME Jews, and these particular Jews had a loud voice that the Romans noticed because it was the voice of RESISTANCE which led to uprisings and conflicts that they had to deal with.
4. We must move to more complex understandings of the historical situation. This is tough for us because we want things to be simple. But they are never simple. Just look at Judaism and Christianity today. Look at the range of ideas and the range of reactions to them within the communities. The ancient world would have been no different, except for the fact that each community had less knowledge at any given moment about what other communities were doing around their world. So insular developments of traditions should be expected to some extent.