The theme tune that played during the opening sequence of every episode, and later during the ending credits from Season 2 onwards.
The theme song "Chan's the Man" by Wheatus, that played during the ending credits of Season 1 episodes. It was replaced by the opening theme for some reason though.
The actual orchestral score from recorder Christopher Ward (who was part of the team that recorded the Phil Collins music from Tarzan) deserves a good deal of mention, especially the Demon Netherworld theme and the overall theme.
The Demon Sorcerers are very popular antagonists. It helps that despite having only a few episodes each (save Shendu), they all have distinct personalities and decent characterization. Several are generally more popular than others, but they all seem to have some kind of individual fanbase.
Jade's Super-Powered Evil Side, the Queen of the Shadowkhan, only appeared in a filler episode in season 2, and was referenced a couple times in Season 4, yet she's incredibly popular. There are plenty of fanart of her and many fanfics where Jade turns again into the Queen (or was never restored back to normal).
There's also a lot of pairing with Jade and Drago. It's based on the bickering between him and future-Jade. It almost sounds like something between jilted lovers. He also singles out present-Jade quite a bit during the last season.
Jackie/Valmont fic is probably second in popularity after that couple. Followed by Jackie/Viper, Jackie/Finn, Valmont/Finn, and Valmont/Jade (with either child Valmont or adult Jade).
The relationship between Shendu and Drago and the final scene of the series with them wrestling in the Netherworld may be seen as unfunny after the real Jackie Chan's relationship with his own son Jaycee became strained following Jaycee's arrest on drug charges and temporarily being disinherited from Chan's worth (Chan is currently the highest paid actor in the film business alongside RobertDowneyJr. and VinDiesel), along with the reveal of a daughter he had from an affair.
Jackie's relationship with Jade, and possibly Shendu's relationship with his siblings, may fall under the same category for the same reasons. At the very least, Shendu's example is Harsher in Hindsight.
A meta example. Jackie Chan and Julian Sands would later go on to play opposing characters in The Medallion (in fact several reviewers called it a live action version of the show).
Also, a minor yet memorable recurring villain was this show's version of Sun Wukong, the Monkey King, who was a Joker trickster that caused a lot of pain. Chan himself would voice Master Monkey in the Kung Fu Panda movies (with his son Jaycee voicing the younger Monkey and his stand in actor, Chow/Shendu voice James Sie, voicing Monkey in KFP's TV series), who has traits of Sun Wukong in his backstory and has a brother who is like Wukong.
Uncle beats himself up over letting Tohru be taken by Shadowkhan and is willing to let himself be engulfed by dark magic to save him, as well as (in another episode) swallow his pride and call a psychic for help when he can't find the ingredient that will wake Tohru up.
Uncle (in front of phone): *casts Tohru wistful look* "For you...my apprentice."
Likewise, Tohru is very protective of Uncle and hates to see Uncle upset. And when Uncle returns to the team in "Samurai Ratso", he's positively elated and cries touched tears. In the same sense, Uncle has their blowfish kiss to share the magic.
Finn, Ratso and Chow have a very close three-way dynamic. They're never apart, never show any interest in anyone outside their trio, always look out for each other...heck, when Finn decides to quit The Dark Hand and start his own crime syndicate, Ratso and Chow immediately quit too so they can keep working with him.
Ratso: "After you quit, we told Valmont he'd better give us raises, or we quit too!"
Finn (thrilled): "And now you guys wanna work for me? That is so c-I mean, that's cool."
Paco's starry-eyed devotion to El Toro seems a bit over-the-top. The fact that he seems to not have any parents and even travels with him doesn't help. On the other end, El Toro also does not help by referring to Paco so affectionately all the time. "Mi Pacito", indeed.
Jackie sees red and loses his cool only when Captain Black has been injured by the bad guys. Then he recklessly goes out for revenge.
Jerkass Woobie: The Dark Hand gang as a whole. Their life of crime never pays, they rarely (if ever) win any fights against the heroes, and they keep getting conscripted or enslaved by evil wizards and demons. Eventually by Season 5, they've gotten tired of all these humiliating criminal/supernatural misadventures, so they decide to settle back into boring yet normal lives.
Tarakudo is the most well-spoken, personable and collected of the Arc Villains and the least harsh taskmaster towards the Dark Hand. He doesn't react to defeats as badly as the others, being instead able to find ways to turn unexpected turns into advantages (like initiating the Oni Mask-wearing Captain Black's corruption by tricking him into summoning his Shadowkhan). In the 4th season's finale, he's revealed to have all along played a Xanatos Gambit that goes like this: whatever the Dark Hand or the Chans collect all nine Oni Masks, they will free his Generals by bringing all the masks in the same location, with only him knowing of the consequences. The heroes discover a way to defeat him — the mask made specifically to seal him — only by accident and even then, Tarakudo is revealed to have hidden the mask in the Shadow Realm, which is accessible to humans only by using the Shadowkhan to take them there. The J-Team gets the mask at the cost of Tohru being corrupted by Tarakudo's mark. Before Tarakudo is sealed inside his mask, he's given a physical body and stripped of his telekinetic powers. He faces the J-Team with martial arts and single-handedly defeats Jackie, El Toro and Viper. Only by distracting him they are able to slam his mask on his face.
Valmont, though only in the 1st season. By that time, he's presented as a sophisticated and good-looking crime lord who seemed to be able to remain in control in most of the situations. Whenever he appeared, he remained cool and gelid snarky whatever addressing his henchmen, his enemies or Shendu. He was also able to react to misfortunes better than in the later seasons. In both of the only times he personally faced Jackie, he fared better than his henchmen. He came up with a Xanatos Gambit, which included poisoning Jackie with venom that turns its victim to stone: either the Talismans stored in Section 13 are handed over to the Dark Hand or the heroes lose Jackie, which would tip the balance in the Dark Hand's favor (which is something Jade notes to Jackie who's selling himself short). He was also smart enough to notbring the antidote anywhere close to Jackie prematurely. After Shendu betrays him and he's arrested, he's revealed to have as a false tooth a homing beacon that allows his Sword Cane to levitate to his position. Sadly, Valmont never demonstrates anything this impressive in any of the following seasons.
When Shendu refuses to pay what he promised to the Dark Hand after they restore him to his true form, Valmont orders Tohru to attack the dragon demon despite it being obvious he can't win. After Tohru is thrown into his apparent death, Valmont shows no remorse for sacrificing his loyal top enforcer who has endured his boss' beratings for the whole first season.
The main villain in "Pleasure Cruise" nearly sunk the ship when he ordered his men to blow open the vault.
Ron the Death Eater: Jade is an interesting case, in that it's not actually her haters that do this to her, but rather she has a lot of fans who really liked her as the evil Queen of the Shadowkhan. There's a lot of fan fiction that have her change back permanently.
Jade got this at first, but familiarity helped to dull it making her either a Breakout Character or Creator's Pet depending on the viewer. At the least, any trouble-making caused by her interference she either fixed herself or managed to make her mistake into something useful, so she was rarely The Load. There were other times when she also played the Big Damn Hero whenever the other characters would go off on their own though haters simply see this as shoehorning. She also tended to be the most wily hero coming up with clever solutions the others never considered.
The Ice Crew was not well-received by viewers, especially when they replaced Finn, Ratso, and Chow in the last season. Ice probably gets it the worst because of the way he talks.
Tohru's Mother at least in her debut episode for treating Jade like a child, and being all around rude to Uncle (though at least Uncle was capable of firing insults of his own) and for initially berating her own son (i.e. scolding him for not owning a car and making her "walk in a typhoon" despite the fact that Tohru was holding an umbrella over her head keeping her dry while he was soaked).
That One Boss: The GBA video game is a pretty standard beat 'em up by Activision (the same studio that later did The Legend of Korra games) except for the four Demon Sorcerer boss fights at the end of stages 4, 6, 8, and 10, all against Wind Demon Xiao Fung, Moon Demon Tso Lan, Water Demon Bai Tza, and Fire Demon Shendu, in that order. These fights do throw a loop at the player when they get to them; running in just attacking will send them to a grave. Plus the last three all summon Shadowkhan backup (for Tso Lan and Bai Tza, it's advised to keep K.O.ing the ninjas until they stop coming before doing anything stupid, but Shendu can summon an unlimited amount that wear blue sashes and throw ninja stars at Jackie from across the room).
Tso Lan is difficult due to a cryptic clue about the background that actually means the player needs to wait under the Moon is covered in clouds (the GBA screen darkens when this happens) before they can attack Tso Lan.
Bai Tza will float away from the player when they advance, and pursuing her too vigorously will lead to them walking off the other side of the pier you're on and into the moat below (Chan can't swim in this game; water counts as the classic Bottomless Pit like in the 3D Sonic the Hedgehog games)
Shendu, who is the Final Boss, (and is the second opponent in a double-boss battle to conclude the game), can bite if you stand right next to him plus summon some nasty spells.
Valmont and his mooks are the bad guys in the first season, and a Shendu-possessed Valmont is the Big Bad for the second, but it sets in with a vengeance during the third season, thanks to the first two seasons draining all of Valmont's capital. Valmont's goons split after that, and Valmont became a washed-up has-been dreaming of former glory. In his last major appearance, he was nothing more than a gag villain who was accidentally mailed to Mexico. In the end of series episode, he's reduced to being a bus driver.
Notably, Valmont had been able to hold his own against even Jackie in hand-to-hand combat during their first confrontation. He has never displayed anything close to that level of combat since then.
The Dark Hand as a whole was introduced as a Nebulous Evil Organization worthy of the attention of the super-spy agency Section 13, and originally had many members and a wide range of resources and connections. By the end of the season it's down to 4 men, Hak Foo making five the next, and eventually just becomes Finn, Ratso and Chow
Hak Foo, too.
The Shadowkhan and Dark Chi Warriors suffer tremendously from this, to the point where they can hardly be regarded as much more than an inconvenient annoyance.
The Shadowkhan play with this a bit; they become a global menace in the 4th season as the ancient evil army of a demon Evil Overlord, and there are now several different tribes of them with unique abilities. On the downside, this makes the classic ninja-Khan seem Overshadowed by Awesome.
Tchang Zu, the Thunder Demon. Despite being one of the most vocal about his loathing of Shendu initially, he is later reduced to being The Voiceless and the demon with the least amount of focus.
The nine Oni Generals. Out of all of them, we are only given the name of one of them, and next to nothing is known about their personalities.
Five of them. In a filler episode of the last season, one of the B-list villains of the show, Bartholomew Chang, assembled an Evil Counterpart team to the J-Team, which he dubbed the Chang Gang. The members of the gang were Philip Crane, Bob "the Bopper" Halfcock, Helga Sorensen and "Little" Tony Chung. Each member was individually at least as skilled as their good counterparts. However, the episode involving them is about Jade screwing up a magic spell and turning everybody into children. After this episode, the gang is never seen again and there is no indication that they or Chang were turned back.
At the end of his namesake episode, the villain Origami swears revenge on Jackie Chan. But he never appears again.
In the Season 3 finale, Daolon Wong revives Shendu in exchange for Shendu's power of combustion. Shendu cheats Wong by absorbing the power himself and also steals Wong's powers of Levitation and Eye Beams. Wong then summons Finn, Ratso, Chow, and Hak Foo, gives them battle armor and tells them to attack Shendu for backing out of his promise. It seems like we'll see either a 3 way fight between Jackie, Shendu and Daolon Wong, or at least a battle between Shendu and Daolon Wong, only for Uncle and Tohru to turn the Dark Warriors back to normal and take away the rest of Daolon Wong's magic.
Tarakudo's Villain Takes an Interest moments with Jade. Though this would've been a good chance to bring back her Queen of the Shadowkhan transformation from Season 2, nothing came of it and as the season went on, Tarakudo eventually stopped bringing this up.
In a filler episode of Season 4, we have Finn, Ratso, and Chow turning over to the good side, after tiring of constantly being defeated by Jackie and getting enslaved by dragons, dark wizards, and floating demon heads. Unfortunately, they are back to a life of crime by the end of the episode. They would eventually make a permanent Heel–Face Turn and lend their help to the heroes later on in the series (this time joined by Hak Foo)...but this happens in the last episode, rendering any potential story ideas stemming from this a case of What Could Have Been.
The introduction of the Oni and Japanese folklore would have been the perfect jumping board for enemies and adventures centering on not just Japan, but other cultures and myths as well. Sadly, the writers opted for going back to adventures involving the Demon Sorcerer legends.
In Season 5, Jackie retrieves another priceless artifact, and runs into a group of his enemies from the Season 2 filler episodes, consisting of the pirate duo from Glove Story, the treasure hunter from Scout's Honor, the black market archaeologist from The Deep Freeze, and the monk from The Lotus Temple. Only for Jade to save Jackie once again, and while nothing fatal happens to them, they're never seen again.
The Woobie: "The Lotus Temple" introduces Xu Lin, a little girl who has been cursed into a monstrous form and trapped inside a lost temple in the middle of nowhere. She just wants to go back home to her family.