It would be a deep dive that I don't want to get into completely right now cause I got other work to do, but probably the central biggest thing he did was further advance and solidify Anselm's notion of penal substitution atonement, a very limited way of viewing Jesus's death that was eventually elevated in Western theology to basically the central dogma of Christianity (and unsurprisingly even moreso in the Protestant churches than be the Catholic church itself). I don't have my relevant books here so I can't quote specifically but I (and many others) feel that Anselm/Aquinas has a very human-warped view of salvation influenced by the structures of their own feudal society, and much of it has been rejected or pushed off as overreaching by Eastern/African theologians.
In terms of logic, one of the central differences in thought between the African/Eastern churches and certain Western thinkers is the comfort level with mystery, the ability to hold multiple principles in tension or to understand that we don't know everything of the mind of God. To many Eastern theologians, a general truth stated about God can be understood to be generally true and informative even if we don't know how it applies to some exceptional case. We can believe something to be "generally true" even if there are exceptions we don't understand. But certain Western thinkers like Aquinas appear to me to treat the Bible like a scientific formula, and rather than hold multiple principles in tension or accept that some things are difficult for humans to understand compared to the all-encompassing mind of God, they have to apply human logic to break down every word and phrase and potential contradiction, which ends up getting them even further from the truth because they force the text to prove things it was never saying. For example, making up the separate, unBiblical categories of "Providential Will" and "Moral Will" to explain God's desires or creating the categories of "ceremonial law" and "moral law" to divide the Old Testament in ways it was never meant to be divided (I don't recall if Aquinas himself used those particular phrases, just using them as examples of the kind of issues that Western theology has often led to.). Because they are uncomfortable with ambiguity/mystery, because they insist on things like, "Every word of the text is infallible!" to mean that it has be to categorized and broken down in a very human manner, they end up becoming more enslaved to their own definitions of words than to the nature and character of God's own self, and create an image of God (which reached the fullest, worst form in John Calvin's Reformation Theology and modern-day followers like John Piper, Douglas Wilson, etc.) that bears no relation to the God I see when I see Jesus Christ.