Luke is not like other action/adventure protagonists. He doesn't save the day by being the strongest, most powerful person. He doesn't kill the emperor in a blaze of glory. Instead, though he is tested and briefly gives in to his anger, ultimately he rejects the cycle of violence and hatred the sith and indeed, his own father were trapped in. He is a Jedi, like his father before him. And ultimately it is Luke's compassion for his father that leads the way for the emperor's downfall. Luke had faith and reached out to the inner goodness that not even Anakin knew was still there. And the emperor is destroyed by the one thing he can never understand, love.
And the thing is, this isn't even necessary to save the day. Without Luke's help Han and Leia still disable the shields. Lando still destroys the death star. Aboard the death star what Luke ultimately wins is a spiritual battle for his and his father's soul.
