
In the Emperor's Throne Room aboard the second Death Star, when the Emperor is mercilessly zapping Luke, one might notice that it takes a really long time for Luke to die. In fact, he never dies. He's even well enough to carry a giant metal man halfway across the Death Star.
If the Emperor is so powerful, so angry, and so strong in the Dark Side, shouldn't he have been fully capable of (excuse the gaming term) one-shotting a freshly homeschooled Jedi?
At first, the Emperor just shocks him for a while, then he finally says his line: "And now young Skywalker, you will die," which implies that he wasn't trying to kill him before, just torture him. But it also implies that after that, he just going to get straight to the killing. And it STILL takes a long time after that. In fact, some of the Emperor's expressions could be interpreted as frustration that Luke isn't dying yet, that the Emperor can't kill Luke just like that.
Why? How?
What was Luke thinking? He was writhing in pain, but he wasn't concentrating on pain. He wasn't concentrating on anger towards the Emperor. He had no negative emotions, he was looking to his father. His "father" who was watching him be tortured by his Master. Luke was pleading for help from someone he knew was still there soley on feeling and intuiton.
Luke had hope.
Evil begets evil. The Dark Side of the Force needs negative emotions not just to do things like conjure lightning, but to have that lightning be fatal. Look back to Attack of the Clones, and the Yoda vs Dooku fight. Dooku uses lightning, and like all Sith, beleives that his anger is all powerful, but Yoda faces passion with serenity, and the lightning is controlled.
In Revenge of the Sith, the Emperor permanently disfigures himself with his own lightning. Yet Luke is unscathed in the end.
Dark Side users think they're the best, but serenity is just as powerful a defense as passion is an offense. Did Luke unknowingly employ that idea, saving his life? It would simultaneously prove that the light side has even more power, by overcoming the Emperor's grip over Vader.
An interesting analog that viewers of Star Trek: The Next Generation may recognize is that of the Psionic Resonator (episode: Gambit, Parts I and II), an ancient Vulcan weapon that amplifies the negative emotions of the user to transmit a deadly force. At the climax, a vulcan seeking it uses it to kill some mercenaries, but the crew of the Enterprise remain unharmed because since hte device relies on negative emotions, if the target remains calm, they survive unharmed.
Okay, so that's kind of an unecessary extra-genre tangent there, but it's too analogous to go unmentioned (but I think it would make more sense if you've seen the episode. I sure hope someone else has).
Why didn't Luke die, or even suffer any permanent damage from extensive exposure to a Sith Lord's lightning?
Because he faced it like a Jedi. He tossed away his weapon, he cast aside his aggression, and that saved him. Had he fought the Emperor, he would have lost. Such is the power and nature of the light side.