Alternative Character Interpretation: Did Vader already know that his son is still alive and none other than Luke Skywalker since the beginning of the movie, or did he not know about it until the Emperor informed him? His only reaction when the Emperor told him about Luke is an approximately-one-second pause followed with "How is that possible?" That smacks of Double Meaning; either: a) He didn't know and was genuinely surprised when the Emperor told him that his son is alive, or b) He knew that his son is alive and attempted to hide it from the Emperor because he planned to overthrow the Emperor if he managed to convince Luke to join him, and was worried upon learning that the Emperor already knew. Since Star Wars: Darth Vader reveals he knew from Boba Fett that their enemy is named Skywalker (whom Vader silently identified as his own son), it's probably the latter.
Award Snub: Many feel that Frank Oz's portrayal of Yoda was very underlooked, between his voice acting and puppeteering work. George Lucas attempted to campaign for him to get a Best Supporting Actor nomination at the Academy Awards, but the voters ultimately felt that it didn't qualify as a "real" performance.
Yoda's theme playing as he raises the X-wing out of the swamp.
The suspenseful music that plays as Luke sneaks through Cloud City isn't awesome in the usual sense, but is very effective.
Continuity Lockout: While the opening crawl does explain the general events of what's going on and it's not too hard to follow the plot, this film does have a few plot points that would confuse newcomers who didn't watch A New Hope, such as the context of why Obi-Wan appears to Luke as a ghost and Luke's initial training with him, and why Han leaving the rebellion to pay off his debt is such a problem with Leia.
Director Displacement: As noted on the main page, George Lucas was the executive producer, and neither directed nor wrote the final script for the film. Some fans like to point that, arguably, the best Star Wars film had the least input from Lucas, an impression which revived after the announcement of The Force Awakens. This neglects the fact that Lucas was solely responsible for the central twist of the film, which he wrote, after rewriting the first draft.
Boba Fett. He had no more than four lines in the entire series and to this day is highly regarded as one of the most popular characters in the entire series, due to his mysterious nature, and for being able to get semi-huffy with Darth Vader and live (and, of course, his awesome armor).
Admiral Piett. Originally a one-off character, he was brought back for Jedi thanks to fan requests.
Kenneth Connelly: "My character was not originally in Jedi. But George Lucas said, 'Ken, I've got a whole bunch of letters from people wanting to know more about Admiral Piett, and I want to put him in the movie. I have no idea what you're going to do but will you do it?"
The pilots of Echo Squadron have become quite popular in the EU, especially Wes Jansen and Derek 'Hobbie' Klivian. Not to mention Wedge Antilles, who was already an Ensemble Darkhorse from the first film.
The other bounty hunters qualify, too. At least Fett was in more than one scene; Dengar, Bossk, IG-88, 4-LOM, and Zuckuss just stand around looking cool on the bridge of the Executor and nothing else. This was all it took to get them all extensive backstories in the Expanded Universe.
Captain Needa, appearing in all of two scenes (and alive only in the first), won a lot of admiration for going to Vader in-person and taking full blame for losing the Falcon. It was clear that he knew he was going to his death, but he went anyway to protect his crew.
Epileptic Trees: Between the release of this film and Jedi, there was a lot of arguing from fans over whether Vader had lied about being Luke's father (one of the proponents of this was none other than James Earl Jones). Lucas included the scene of Yoda confirming it in the next film specifically to kill any doubt.
Why did Lando order the evacuation of Cloud City? Was it only so they could escape the tightening clutches of the Empire while they still could, or was it to make it harder for the Imperial troops to stop the heroes' escape by giving them a chance to slip out amongst the confusion of the general evacuation?
This is the only movie where Luke carries a "blade" and a blaster. It also symbolizes the fact that Luke's torn between two worlds: The Rebellion hero, and the Jedi. When he chooses to take the Jedi route, when fighting Vader, he goes from "blade and blaster" to just using his lightsaber. In A New Hope he mostly used the blaster, and in Return of the Jedi, only the lightsaber.
Every time we see Luke training under Yoda, he fails a test:
Looking for a "great warrior," Luke fails to recognize Yoda when they first meet.
Luke ignores Yoda's suggestion to not take any weapons into the cave.
Luke can't conceive of how the Force could lift a heavy X-Wing, and fails.
When his Force sense tells him his friends are in danger, Luke doesn't listen to Yoda or Obi-Wan's warning that it's a trap and that he's unprepared.
It's a blink-and-you'll-miss-it moment, but notice that Luke activates his lightsaber before Vader in the movie's final showdown. It serves as a perfect highlight of his impatience, aggression, and continued flirtations with the dark side.
While not really touched upon, Luke directly deceives his Rebel allies when he tells them he'll meet them at the rendezvous point while knowing full well he plans to go to Dagobah. It's kind of a dick move (and in real life would count as going AWOL in wartime, a deadly serious crime) and foreshadows that Luke might not be as Jedi as he thinks.
In their lightsaber confrontation, Luke holds his weapon firmly in both hands while Vader starts off one-handed until he springs the carbonite trap. He's so overconfident that he doesn't think overcoming Luke might require his full dueling skill.
The carbonite facilities that Vader describes as "crude" lack OSHA Compliance because they can. The employees are the small Ughnauts, refugees from the Anoat System, a planetary system that went bankrupt and entered anarchy so horrible that the Empire refuses to touch it because it's just not worth the effort. Plus, the facilities operate outside of any governing body, excusing them from worker protection laws. Lando has the humans who probably have chemistry or business degrees doing the white-collared jobs with comfy office chairs, and the jobless unskilled foreigners who don't speak Basic doing the manual labor without safety precautions. Pretty shady for a place that seems so utopian.
The bridge crew on those Star Destroyers must be traumatized by their commanding officers frequently getting executed in front of them, and even worse, their bodies are just left laying on the floor for an indefinite amount of time.
The gravity on the asteroid they land on should be incredibly low. When they are in the Falcon, it can be explained as Artificial Gravity, but later they go outside to investigate and the gravity is still normal. Even if it were a large asteroid, for example, Ceres only has 1/36 the gravity of earth. Gravity aside, another problem is that they go out into the cave without any space suits or protective gear other than oxygen masks. Unless that asteroid has an atmosphere of some kind, this doesn't make a whole lot of sense.
The can of worms that is Han's line, "Then I'll see you in Hell!".
"Funny Aneurysm" Moment: Before the asteroid chase, Han is trying to repair the Millennium Falcon when his toolbox falls on top of him, hurting him in the process. In June 2014, Harrison Ford got injured during filming for Episode VII when a hydraulic door from the Falcon set fell on top of him.
Not only that, but as she's being pulled away by the Imperials, she screams once more, "Traaaaaaaap!!!!"
In Return of the Jedi Leia says "I know. Somehow I've always known." when Luke tells her that she's his sister. So, about that kiss on Hoth, Leia...
The scene where R2 and Yoda fight over Luke's lamp was made even funnier when the prequels revealed that ESB was not the first time Yoda and R2 had met. It's somewhat comical to think that R2 probably realises that he's dealing with one of the most powerful beings ever to have lived... and decides to continue trolling him anyways.
Han's "I know" to Leia's "I love you" was improvised by Harrison Ford, as he felt Han's scripted line "I love you too" was out of character. In Working Girl, Ford is the one who says "I love you," to which his love interest replies "I love you too."
Han's kissing Leia before being frozen in carbonite, while Darth Vader is present in the same room, is funnier after Leia is revealed to be Luke's long-lost sister and, therefore, Darth Vader's daughter. This means Han kissed Leia in front of her dad!
When he arrives on Dagobah in search of the wise Jedi master Yoda, Luke encounters a small green muppet who speaks in a funny manner. The film is clearly written with the intention of this muppet's identity reveal being a big surprise to first-time viewers... except that, since the film's release, everybody knows who Yoda is.
Magnum Opus Dissonance: A lot of Star Wars fans consider this to be the best film in the entire saga, but Lucas actually considers it the worst. Yes, that's right, the worst of all six movies.
Lando betraying Han. note Darth Vader beat Han to the punch of arriving to Cloud City. Lando was left with a Sadistic Choice of either selling out Han or allowing Cloud City and all of its inhabitants to be destroyed.
Its Atari 2600 game, which was not only pretty complex for a 2600 game, but also is considered one of the best licensed games on the system. Then fast forward to the mid-1990s and the Super Star Wars trilogy games.
The arcade game, made using the same graphics and the same engine as the previous Star Wars game.
C-3PO reached the level of scrappiness in this movie due to him gaining characterization by being a shrill worrywart that never shuts up and constantly rattles off unfavorable statistics about their current situation. He also can't keep himself from complaining to Chewie and R2-D2 when they repaired his dismantled body. However he is remembered much more fondly ever since the prequels introduced a certain Gungan.
Admiral Ozzel is an In-Universe example of this, with the novelizations establishing that the man was notorious for being shortsighted and not thinking through his strategies like his fatal maneuver at Hoth.
Squick: After Return of the Jedi, Leia kissing Luke on the mouth became this. Even though she states in Jedi that "somehow" she had "always known" that she was Luke's sister, she may or may not have done it to make Han jealous (in which case, that may or may not be an implied Retcon).
Shocking Swerve: At the time, Darth Vader telling Luke that he was his father. It came right out of nowhere and the foreshadowing from the previous movie could have referred to a different kind of father. It worked, nonetheless.
The Tauntaun shots have not aged well. The life-size puppets are fairly convincing, but the shots of them running are recognizably stop-motion.
The Wampa was mostly removed because of it in the original version. The Special Edition inserts newly filmed scenes of it.
During the AT-AT battle, the Snowspeeders (and related shots, as per Word of God) were not printed at their full opacity. This is evident in a view from the cockpit. Like the Wampa example, this was eventually fixed.
Luke goes a bit transparent when he falls down the Cloud City shaft.
When the Mynocks start swarming after Leia and Han, you can clearly see they're large sheets of plastic tied around poles because the head is completely absent from them, unlike the previous shots of them. Even Irvin Kirschner admitted that the reason they cut away from the Mynocks so quickly was because of how awful the props looked.
Suspiciously Similar Song: The Imperial March sounds a great deal like Prokofiev's "Montagues and Capulets". Not coincidentally, the two songs are juxtaposed on Epica's album The Classical Conspiracy.
Tear Jerker: Han's poor tauntaun freezing to death in the first act. Necessary, yes, given that its body saves Han's and Luke's lives — but still heartbreaking.
They Changed It, Now It Sucks: In the George Lucas Altered Version, Boba Fett's voice is probably the biggest point of contention; while Temuera Morrison gives a decent performance, many people grew attached to Jason Wingreen's original voice acting as the character. However, fans find this to be a minor gripe compared to what the other two original trilogy movies have to put up with.
Mostly averted, however, with replacing the original Emperor hologram with Ian McDiarmid, as not only does this maintain tighter continuity, but McDiarmid simply is Emperor Palpatine to many, and his performance is seen as much better than the rather monotone voice-over Clive Revill provided in the original version. The fans who complain about this change are actually in the minority.
They Wasted a Perfectly Good Plot: Boba Fett is the only bounty hunter seen after Vader puts a price on Han. It would've been interesting to seem some competition between them for who can capture Solo.
Poor Captain Needa. He clearly knows he's going to die but he tries to apologise in person on the small chance Vader will be merciful and/or take the blame for losing the Falcon (which has more to do with the daringness of Han Solo than any tactical shortcoming on Needa's part) out on him personally rather than the whole ship. (It does not help when we learn, in the EU, that almost his whole family was executed shortly after his death.)
Piett. The newly promoted Admiral is fully aware of his own officers that Vader keeps killing, and the poor guy has this look on his face that screams "Oh god, I'm next." Ironically, Vader lets him live all the way through the movie and well into Return of the Jedi. In fact, when Piett does finally die, Vader has nothing to do with it. This is probably due to Kenneth Colley's excellent silent acting in many of these shots. His expression as he listens to Vader grant him his You Are in Command Now promotion while very deliberately trying not to look as Admiral Ozzel is choking to death in the background is comedy gold. Unlike the generic officers in the previous film, Piett clearly has a personality.
Vindicated by History: The film, while commercially successful, was not as popular as A New Hope when released, and Return of the Jedi was seen as a return to form after this. However, the films then started to be released on home video, and those watching it there started seeing this as better than the other two. Now it's considered the best film not only in the original trilogy, but out of all the Star Wars films in general.