Alternative Character Interpretation: Is Tuco guilty of all those crimes that's being read off just before he's hanged, or has he falsely admitted to these crimes to drive up his bounty, and make his and Blondie's scam more profitable?
Awesome Music: "The Ecstasy of Gold" and "The Trio" play a large part in creating one of the most memorable finales in the history of film. The former in particular is so memorable that Metallica have used "The Ecstasy of Gold" to open their concerts since 1983.
Broken Base: the numerous changes made to the movie over the years tend to breed a fair bit of debate.
Is the American theatrical cut, which runs 16 minutes shorter than its Italian counterpart, a better movie because of its tighter pacing, or an incomplete experience with a few too many unanswered questions?
Does the cave scene belong in the movie? Most viewers say "no" because Sergio Leone himself removed it shortly after the movie's premiere, but it made it into the extended American cut anyway.
Speaking of the extended American cut: should Eastwood and Wallach have returned to dub themselves nearly 40 years later, or would it have been better to use soundalikes, as was done for the late Lee Van Cleef? Is the new 5.1 surround sound mix tolerable or does it destroy the whole movie?
Is the latest blu-ray release, which presents the movie with a strong yellow tint not found in previous releases, accurate to the way the movie was supposed to look in theaters? Supposedly the Technicolor print used as a reference for the blu-ray features the yellow tint, but other surviving prints appear more red, which is how previous transfers of the movie looked.
And of course, the longstanding debate about whether or not this film actually shares any sort of continuity with A Fistful of Dollars and For a Few Dollars More or if the movies are only unified by the idea of the Man With No Name rather than one specific character.
Creator's Favorite: Tuco. Both Eastwood and Van Cleef realized that the character of Tuco was close to Leone's heart, and the director and Wallach became good friends during the making of the film. Van Cleef observed:
"Tuco is the only one of the trio the audience gets to know all about. We meet his brother and find out where he came from and why he became a bandit. But Clint's character and Angel's remain mysteries."
Designated Hero: Outside of a Pet the Dog moment here and there, Blondie's status as "the Good" is... pretty questionable. He mostly comes across well in comparison with his rivals, but he's also emotionally distant and not as personable as his bandido rival Tuco.
Draco in Leather Pants: Tuco. He does almost too good a job of making the audience sympathise with him. Some fans even go so far as to forgive his charges of rape, murder, kidnapping, extortion, armed robbery, etc. It doesn't help that critics have painted him as the emotional heart of the movie.
Evil is Cool: Angel Eyes might be an utter bastard, but damn if he isn't a badass.
Fan Discontinuity: The tie-in novels, which aren't even by the original authors, for Blondie's enigmatic past getting demystified and Tuco suffering a slight case of Flanderization in his only other appearance.
Foe Yay/Ho Yay: Blondie and Tuco, mostly carried through bizarre Does This Remind You of Anything? innuendo. They are really into tying each other up, Tuco bursts in on Blondie tenderly polishing his gun in his hotel room (not a euphemism), and while Blondie is talking to the naked Tuco in the bathtub, he smirks, while suggestively playing with the end of a bedpost. This is almost certainly intentional, as Sergio Leone made them share a bed for the whole of filming as Enforced Method Acting. Quentin Tarantino, who was heavily influenced by the film, claims to ship it a lot.
Fridge Brilliance: While the Mexican Stand Off seems to have an uncertain issue at the first viewing, in an incredibly subtle instance of Show, Don't Tell, Leone is actually showing the public how it's going to end: at first Tuco hesitates between Angel Eyes and Blondie, frantically switching his sight between the two, though it is foreshadowed by a scene that Tuco is more inclined to kill Angel Eyes, while Angel Eyes quickly asserts that Blondie is the most dangerous of the two, and stares at him. Meanwhile Blondie looks at Tuco and without exchanging a single world, with only a single nod from Blondie, they agree to both shoot Angel Eyes. They lure the latter into a trap, only staring at each others to have him drop his guard, Angel Eyes thinking he has an opening against Blondie sneaks his hand to his revolver and tries to shoot him, having checked that Tuco wasn't looking either, but Blondie reacts faster and kills Angel Eyes, while Tuco tries to shoot too, only discovering thathis gun was emptied beforehand by Blondie.
Jerkass Woobie: Tuco. It's somewhat ironic that the most sympathetic and likable character is the one who's most morally ambiguous.
"There are two kinds of people in this world..." and variants thereof.
The title itself has been a fairly common neologism in the English language.
Moral Event Horizon: Angel Eyes crossed the line at the beginning of the movie, killing the son of the man he was hired to kill. It's one thing to kill the man himself, as that was just business ("When I'm paid, I always see the job through."); he didn't have to kill anyone else beyond those he was hired to kill (yes, those–just before dying, Stevens pays Angel Eyes to bump off the guy that sent him to do the deed). Although the boy came running down the stairs behind him, rifle drawn. Angel Eyes was too smart to chance that the boy would be too scared to pull the trigger.
Possibly deliberate. The ridiculously long list of charges read at one of Tuco's near-hangings explicitly mentioned that he was hanging for raping a white woman, but also for having consensual sex with an underage black woman. It's never stated how old she was though, so the 'underage' part might be a factor in the hanging.
In an ultimate example that involves Evil Gloating interrupted with a Talk to the Fist, the One-Armed Bounty Hunter that caught Tuco while the latter was taking a bath. Tuco even mentioned that if he was going to shoot him, shoot. Don't talk.
Tuco himself became an example earlier, when he could have hanged Blondie faster (and tied his hands) instead of just sat there monologuing, but given his obsessive Foe Yay sadism, it's sort of understandable on his part.
Both Angel Eyes and Tuco theoretically could guess the money was at least somewhere near Arch Stanton's grave. Otherwise, how did he know there was an Arch Stanton in the graveyard? Possibly justified in that neither have enough time to really think about before the Mexican Standoff.
Even Blondie isn't immune to a bout of idiocy. When he decides his gambit with Tuco has run its course, he just leaves him out to die the desert instead of just letting him hang the next round. Because of his rule of only killing in self defense, and that he was positive the bandit, with his impatient and impulsive nature, and bound tight with rope, wouldn't survive out in the heat, this mistake almost costs him his life. He clearly took for granted Tuco's admonishment.
Tuco: But if you miss you had better miss very well. Whoever double-crosses me and leaves me alive, he understands nothing about Tuco. Nothing!