Adaptation Displacement: For baby boomers, the 60s television adaptation pretty much completely displaced the original radio series and movie serials in their consciousness. Later generations are familiar with the property primarily through the NOW Comics and Dynamite Entertainment comics series, and have little (if any) familiarity with either the radio series/movie serials or the TV series. This got bad enough that The Film of the Series has a comic-book motif in the end credits.
Ensemble Dark Horse: Thrown all across the board with Kato throughout the franchise. He began as a simple sidekick but over the years, he grew into popularity. Most modern versions have Kato as the muscle while Green Hornet was the central figure. In the movie, everything is taken to the next level. Kato was basically responsible for everything that the team did, from the gadgets to being the muscle. While Britt is still the main character and the one calling the shots, the movie does involve them clashing with each other over who was in charge. Most people agreed that Kato was the best part of the movie, so it's likely they intended the movie to be like this.
Comic Series
Shipping: Britt Reid and Casey Case. Hinted at in the radio series (in at least one episode Sentinel reporter Ed Lowery banters with Miss Case about her harboring a crush on Reid). Subtly made part of the subtext in the television adaptation in at least two episodes ("The Frog is a Deadly Weapon" and "Invasion from Outer Space"), in spite of firm insistence by creator George W. Trendle that the relationship between Case and Reid be kept strictly professional. Made "canon" in NOW Comics's "Hornetverse", where Britt Reid II marries Casey Case after his retirement (Trendle having died in 1972, the subsequent rightsholders apparently discarded any objections to a Reid-Case romantic relationship).
Broken Base: As Michel Gondry himself admitted in an interview, most fans of the property didn't care for this new comedic take on the characters. A small but dedicated group did like it, and actively defend it, however for it being different from the original.
He's Just Hiding: Danny Clear notices how Chudnofsky left behind a briefcase in his office. Given his Genre Savvy nature and the amount of time it takes Chudnofsky to get clear to detonate the bomb, Danny might have realized what was about to happen and fled in time.
Ho Yay: Good God. Some people bring their Shipping Goggles to movies. Some movies forcibly glue them to the viewers' faces.
"I'm surprised you haven't hit on me yet!"
"Shen Di Forever!"
(Psst: it's "Xiong Di" in pinyin. And it actually does mean "brothers", so, umm, let's not go there please.)
Brit: "I want you to take my hand and come with me on this adventure!" Kato: "I'll go with you, but I don't want to touch you."
Kato changed Brit's diaper while he was in a coma. (Granted, the coma was his fault...)
Leonore: "Maybe you two kiss, but I'll have nothing to do with you!"
Jerkass Woobie: Britt may not be the most likable do-gooder, but considering he grew up with an absolute dick of a father who always was almost never there for his son and always told him he was a huge disappointment when he actually was physically present, it's no wonder Britt has a lot of issues. The fact he's struggling with the grief after his father's death also adds to this.
The simple idea of Seth Rogen playing one of the most famous pulp heroes of all time got on a lot of people's nerves.
To a minor extent, Jay Chou as Kato. The main point of contention being that some fans prefer that the role be given to martial artist actors who were probably inspired by the character and Bruce Lee to take up the practice in the first place and give them the opportunity to become a Promoted Fanboy.
They Wasted a Perfectly Good Character: Brit's maid, who seems humorously timid and who he didn't fire (like the other servants) for some reason, could have been a decent recurring minor presence, but only has one scene that lasts less than a minute.
Awesome Music: Ladies and gentlemen, Al Hirt. The piece is not only enormous fun to listen to, but is so fast-paced that the King of Trumpet himself allegedly passed out when the recording was done.
"Alias the Scarf": James Rancourt was a Serial Killer who terrorized the city decades ago, killing numerous victims by garroting them with a scarf in order to join the "great company" of Jack the Ripper, Attila the Hun and Bluebeard in immortality through fame, before disappearing and adopting the identity of an unassuming researcher at a wax museum. After a statue of The Scarf is placed in his museum's chamber of evil, he begins killing again, pretending to be his own wax statue. He starts by attempting to kill his partner and apparent friend at the museum, then rampages through the street, killing a man sleeping on a public bench and trying to kill several more before the police intervene. When confronted by a girl he used to confide in and who loved him he attempts to kill her as well to ensure his "immortality" by killing her, right before the Green Hornet and Kato apprehend him.
"Invasion from Outer Space" two-parter: Dr. Eric Mabouse is an ex-military researcher who now seeks ultimate power. Pretending to be an alien and kidnapping Lenore Case to get Britt Reid and the police to clear the roads, he and his gang steal a hydrogen bomb, with Mabouse fully intending to set it off simply for the power trip. When Ms. Case escapes, he orders her hunted and killed, and sets one henchman up to die with the Green Hornet and Kato in a Death Trap when he suspects him of disloyalty. When the police close in, Mabouse simply decides to leave the warhead outside the city and detonate it to destroy the city once he's gotten clear, and leaves another henchman behind to die simply for objecting.
Draco in Leather Pants: Double Subverted in the second broadcast episode of the TV series ("Give 'Em Enough Rope"), a female lawyer friend of Britt Reid's is captured by the Villain of the Week, and is rescued by the Hornet, in the next scene she's telling the story to Britt and gushing about the Hornet, wonders if he's really as bad as everyone thinks, and says she hopes he never gets caught, and if he does, she'll defend him for free. Subverted in that the Green Hornet really is a misunderstood, maligned hero. Double Subverted since she doesn't know that.
Kato's fighting skill. Bruce Lee's then unseen fighting skills caused many people to over look the Hornet's own abilities which were more mental and less physical, though he was more than capable of fighting back, and people believe that the Hornet did nothing but take credit for Kato's work.
This is extremely bad in Dragon: The Bruce Lee Story, in which a scene of the Green Hornet show is being shot wherein the Hornet is tied up and looking so scared that he seems like he's going to wet himself. Britt was every bit as cool under fire as Kato was, only for Kato to kick down a door and beatdown the villains with seemingly super human strength as the Hornet does nothing more than cheer him on and look pathetic. This never happened in the show; Kato helped sure but the Hornet had to save him a few times, but it now seems that there at the very least has to be a joke at the Hornet's expense about who the real hero is, despite the fact that Hornet is unarguably the smarter of the two and Kato, at the very least the TV series version, would have gotten nowhere fighting criminals without Britt's intelligence.
Hirohito Juuma is the wicked son of Oyabun Oni Juuma who wishes to restore the "family honor" to sate his ego. Murdering Kato's wife and wiping out the other heads of the Yakuza, Hirohito arrives in Century City and murders the original Green Hornet along with multiple others, taking the disguise of the Black Hornet. Murdering his way through civilian and criminal alike, Hirohito reveals his true plan is to hijack a state-of-the-art, remote-controlled stealth bomber to sell to the highest bidder, and prove its power by nuking Century City and everyone within off the map.
Rise/Reign of the Demon: The masked mobster Demone seizes control of the criminal rackets by murdering all who resist and runs rings of forced prostitution. Allowing the Hornet and Kato to rescue several women, Demone murders one of their escapees to get to the masked heroes. Demone is revealed to be working with Nazi fascists to help sow discord in the US, kidnapping innocents and conducting ghastly experiments to turn them into ravenous zombies that will attack and slaughter the police upon command. At the climax, Demone unleashes them upon Century City's police station before settling back to enjoy the show, caring nothing for lives lost as long as he profits.
The piece is not only enormous fun to listen to, but is so fast-paced that the King of Trumpet himself allegedly passed out when the recording was done.