The last act is what gives it dramatic power, for me. The Captain and Maria defiantly singing Edelweiss to the Gestapo agents at that music festival, with all the crowd joining in, is so moving. And the nuns helping them at the end, sabotaging the Nazi vehicles ("Forgive me Reverend Mother, for I have sinned"), is hilariously satisfying. Also, the moment Rolf chooses to give them away rather than coming with them is a brilliant illustration of the poisonous influence of the Hitler youth.