The Empire Strikes Back is brilliant not only as a dark (and risky) sequel to Star Wars but also because of the rich literary traditions it draws upon. The clearest connections that can be drawn are to Sophocles's Oedipus Rex. Obviously, there is the incestuous love between Luke and Leia that develops here, and the way Luke symbolically kills his father. The parallels go far deeper, however.
In discussing a tragedy like Oedipus, one often considers the emotional crux of the story. What is it that makes us connect with the tragic hero? In Oedipus, it is the fact that we admire his greatness yet do not share his fatal flaw. We see ourselves in Oedipus, and know that if we were so headstrong and proud, we would fall just as he did.
Not only is Luke Skywalker brought down by a tragic flaw of Sophoclean scale, he has the same exact flaw. Early in the movie he heads out into the blizzard despite being warned against it, and almost dies as a result. He is too proud to admit he was wrong, and the next time he receives a similar warning he does not heed it. The character of the blind seer, Tereisias, is paralleled by that of Yoda. Just as Tereisias warns Oedipus not too inquire further into the matter of the murder of his father, so does Yoda warn Luke. He then tells Luke not to go to Cloud City. Luke does so anyway. Clearly Luke is too headstrong to ignore his curiosity or to avoid the duty he feels toward his friends, which is just like the duty Oedipus feels toward the city of Thebes.
The most stunning part of both Empire and Oedipus is the way that their heroes fall without really losing anything. The eyes that Oedipus loses and the hand that Luke loses are symbolic; they represent a loss of pride and a newfound humility. The true fall of each character is in his realization of himself. Oedipus realizes not only that he has married his mother and killed his father but also his own weakness as a person. Luke realizes not only that Vaderis his father and that his quest for vengeance is misplaced but also his own weakness: haughty, foolish pride.
I could elaborate more on the stunning parallels between these two great works, but I will leave the rest to you. Go, watch them again and see how the great masters of Western culture have worked their magic.