In studying both the Summa and Bondage of the Will, it has occurred to me that due to the influence of Augustine Luther and Aquinas have essentially the same idea of divine omnipotence and the human will. Both agree that the human will is not some sort of neutral capacity that starts out as a blank slate and then just decides whatever it wants. Human beings have a nature. They act out that nature when they will things. A evil person wills evil freely, though he cannot but will evil insofar as he is evil. Just as an apple tree gives off apples freely, though it is in its nature to do so, so a person acts out their nature without coercion.
The difference comes from what one considers the basis of human willing: i.e. faith vs. love. For Aquinas, the human will is driven by the love of the good. God is the supreme good, but temporal realities are good to. Sin is failing to will God as the supreme good and therefore choosing a more limited version of the good. Nevertheless, humans are still choosing the good, they just are doing so in an inadequate manner. For Luther, the whole issue is faith as trust. God should be trusted above all things, but instead humans trust in other creatures and in themselves.
This means that for Luther sin is much more radical thing than it is for Aquinas. There is something of a zero-sum game between belief and unbelief. One either trusts in God or one doesn't. Trusting in creatures is not some how moving in the right direction. Rather it is antithetical to a proper relationship with God. For Aquinas, by desiring temporal goods one is still sort of moving in the right direction. One is willing the good, but not as well as one could.