-The character development. We see Luke mature from a wide-eyed, childish farm boy into an ambitious young man intent genuinely devoted to taking down the Empire. Han and Leia's romance develops, and their qualities rub off on each other-- Han becomes more understanding and sensitive, and Leia becomes more strong-willed.

-The Luke/Vader duel. Say what you will about the fight choreography, but in the context of the film it makes total sense. Luke has next to no experience using a lightsaber, and Vader is really only toying with him because his intent is to leave him alive. The lighting and general set design is fantastic as well, and, of course, the reveal that Vader is Luke's father is done perfectly.

-The tone it sets. Empire is without a doubt the darkest movie of the OT, with a general tone of defeat. From the loss at Hoth to the moment where Han and Leia realize they've been played by Lando to Luke's loss to Vader, the movie is rife with emotion. We see a very genuinely portrayed romance blossom, and we see the beginnings of Luke's evolution as a human being. The ending is done perfectly, with a dark tone with a few slivers of hope. Lando helps Leia and Chewie escape Cloud City so as to formulate their plan to save Han, and Luke gets a robot hand to replace his old one, and he's ready to learn to become a Jedi.