During the battle on Hoth, Luke single-handedly takes down an Imperial Walker, by himself. At that, you'll understand why the Emperor is so concerned about the kid; as in, "If the boy can do that with barely a few days instruction from Obi-Wan Kenobi, what could he do once he is properly trained?"
And of course the Walkers themselves.
During the same battle, the Rebels manage to disable a Star Destroyer with the Ion Cannons, creating a gap in the Imperial blockade for the Rebel ships to escape through.
Little Yoda lifts big X-Wing.
Luke: I don't believe it.
Yoda: That... is why you fail.
Luke Skywalker VS Darth Vader.
Above all, when Luke is clinging to that ledge, thoroughly beaten by Vader both physically and emotionally. Luke willingly defies Vader's offer - "Come with me, it is the only way." - and steps off the ledge. Not even knowing if he would survive the fall (he does, barely).
For Vader, when he starts picking up random pieces of machinery and tossing them at Luke in the middle of the fight. That's when you really understand that, yeah, Luke's pretty handy with a lightsaber, but he iswayout of his league here.
Compounded by the realization that Vader isn't even trying to kill Luke. He wants him alive but beaten to within an inch of his life, so as to be vulnerable to his suggestion to join forces. Imagine how brief it would have been if Vader had just wanted Luke dead.
The fact that, despite the handicap Luke faced, he actually managed to land a blow on Vader's shoulder, though the armor stopped it.
Also, when Vader kills Admiral Ozzel. Ozzel isn't even on the same ship as him, let alone the same room, and yet he easily force chokes him; distance is no object to Vader.note Although more to the point, distance is no object to the power of The Force.
When the Millenium Falcon's hyperdrive finally works thanks to R2.
Wedge and Wes being the only Rebels to actually take down an AT-AT with their tow cables.
Boba Fett doesn't even flinch when Darth Vader gets in his face about the disintegrations.
How about the fact that Boba is able to get semi-huffy with the Sith Lord and live!
Han Solo after being tortured. When he finds out that this was all to lure Luke into a trap, he uses a bit of strength he has left to punch Lando before being restrained. Lando was in the middle of a Heel-Face Turn at the time, but the fact that Han's best friend putting his other best friend in danger being the last straw for him was pretty awesome.
As Han Solo is lowered into the carbonite freezing chamber, Leia declares her love to him. She doesn't get the response she was hoping for.
Leia: I love you!
Han: I know.
The big reveal, in some of James Earl Jones's best vocal work.