Anti-Climax Boss: Iosef doesn't even put up a fight, going down with a single point-blank bullet to the head, which fits with his characterization. Played with in that he turns into a Disc-One Final Boss; Viggo kills Marcus in retaliation, and puts up far more of a fight when John comes after him.
Catharsis Factor: Do you absolutely love dogs? Want to hurt anyone who hurts dogs? This is the movie for you.
Fanfic Fuel: Many fans thought the setting was so well built that the movie must be the result of Adaptation Displacement, but no; it's an original property all the way. The Continental and the universe built around it is just begging for stories to be told. Even Keanu thinks so.
Fan-Preferred Couple: Although she only appears briefly, some fans think that Addy the bartender would be a cute and healthy fit with John (once he gets over Helen's death, of course) since she's a genuinely sweet woman who's connected enough to the underworld to not be bothered by John's career but would also keep him away from the most unpleasant parts of the lifestyle (since you're not allowed to conduct business on Continental grounds.) Being played by the gorgeous Bridget Regan doesn't hurt for popularity either.
He Really Can Act: Keanu Reeves uses his understated delivery to convincingly portray a terrifying manifestation of Tranquil Fury. Not to mention the scene where he progressively broke down in tears as John read his late wife's letter explaining the newly arrived puppy as well as his Badass Boast when he's captured by Viggo.
Variations of the "Yeah, I'm thinking I'm back!" phrase are becoming popular amongst the movie fanbase.
"He was the one you sent to kill the fucking boogeyman," is popular for describing John due to its conciseness and sheer badassery.
Jokes bout John Wick butchering people with a pencil, alluding to Viggo saying he saw John kill three men in a bar with a pencil ("with a fucking pencil"). This only increased after the second movie actually showed him killing people with a pencil.
More broadly, jokes about not messing with John Wick's dogs. Animal lovers also like to joke that his response to his dog's death was perfectly reasonable, or put his picture in response to stories of animal cruelty.
While relatively minor compared to many examples of this trope, killing John's dog put Iosef firmly in the "irredeemable villain" category, at least in John's mind. Ironically, Iosef doesn't really do much besides this one evil act, since most of the time he just lazes about while drinking, smoking, chasing girls, and playing video games. As his father and Aurelio spell out to him, it's not what he did, it's who he did it to. His arrogance and ignorance get to be too much for any knowledgeable person in The Mafiya to bear, just with the one act of offending Wick, who helped build his family's organization.
Ms. Perkins crosses it by killing Harry. Breaking The Continental's rules by going after John there was definitely a dick move, but murdering a man who's already given his word not to harm you, when you could have easily disabled him and escaped, or even simply waited for him to leave (he explicitly states he's leaving her tied up for housekeeping to find), falls squarely into this.
One can almost, almost sympathize with Viggo for trying to get John to back down, since he didn't pick the fight to start with. Once he murders Marcus, though, there's no going back.
Nightmare Fuel: John Wick is utterly terrifying. If he wants to kill you, he will kill you. Especially if you deserve it. No matter how rich or how powerful you are, no matter how many bodies and guns you throw at him, no matter how far or how fast you run, John Wick will kill you.
Officer Jimmy. He's onscreen for barely half a minute and says less than twenty words, but he gets one of the funniest scenes in the movie. He also immediately knows to leave John alone, unlike many other characters, making him a standout Only Sane Man.
Signature Scene: The Red Circle fight. It shows how deadly John is (about a third of his bodycount comes from this one scene), and the use of color and music often prompts comparisons to the club scene from Collateral (itself a Signature Scene for that movie).
Too Cool to Live: Marcus is John Wick's mentor; he's the guy who taught him how to "kill the boogeyman". Someone this cool dies just before the climax so John can fight Viggo alone.
What an Idiot!: Iosef seems to be the only person in the entire criminal underworld who doesn't know who John Wick is, or why it's a bad idea to piss him off. Made dumber because John Wick worked for his father as recently as five years before the film started, so you'd think he'd at least recognize the guy. The point is driven home in that even when his father is clearly terrified about what's to come, Iosef sees no problem in hanging out in a club (and when Viggo starts to explain why Wick is so dangerous, the first thing Iosef says once he's allowed to talk back? He offers to go back to Wick's house and finish the job). Viggo himself exploits him by explicitly using him as bait. He doesn't even seem to process or care why everyone else is so terrified of the guy until he sees John in action.
Perkins also qualifies, Sure, 4 million is nothing to sneeze at even in a world where the most common career seems to be a hitman, but as the ending of this movie and its sequel show, going against the rules of the Continental is the single most stupid thing someone can do in this world outside of pissing off John Wick.
Complete Monster: "Calamity" is a half-feral assassin who, in stark contrast to other assassins employed by the Continental, relishes in deliberately targeting innocent lives for pleasure. Calamity blew up an entire village and slaughtered dozens for the sake of attempting to kill a young John Wick, and is freed from custody years later where she proceeds to murder the people who break her out and escapes to old habits. Calamity fires upon populated buildings to lure John out while cackling, taking innocents hostage and only hesitating on murdering her own gang members for fun simply because "it would be over too soon" that way.