As I've said elsewhere, Sam resists the lure of the Ring because he knows his place in the world. He is a simple gardener and that's all he wants to be, so the Ring's lure of power has no hold over him. His lack of ambition, rather than immense willpower, is his salvation. It is rather an uncomfortable message for a 21st-century primarily-American audience to embrace, since we are conditioned to think of ambition as a good thing. On the other hand, I think Tolkein was coming at the idea of the perils of ambition less from a class-stratification point of view than from the point of view of a Christian theologian (think Lucifer).