These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Codename: Kids Next Door
Kids Next Door Mission
Codename: Kids Next Door is a show where Your Mileage May Vary. Kids may actually like this show, and even teens and adults. However, some may not like it based on its premise.
Crowning Music of Awesome: The rock opera episode, complete with a score by GWAR, who re-performed the song themselves after being unable to find stock copies.
Designated Hero: Sometimes one can't help but tilt their heads at the KND's actions, like in the episode where the agents fight against a rabid, Batman-esque dentist, whose heinous crime is to improve the kids horrendous dental health. On another episode, the kids hijack a delivery truck to destroy Type-P torture devices, a.k.a. pianos. So, what about the kids who like playing? They never stop to think about an hypothetical little Timmy whose only dream in life is to become a famous musician?
In the case of Nightbrace, he turned out to actually be quite heinous because he wasn't even a dentist, but an insane candy shop owner who couldn't stand kids eating the sweet candy that he gave them, so by night he went out to fix their teeth. On that note, he was kicked out of dental school for trying to put braces on babies. Even the real dentist didn't approve of his actions, but by then he was too wrapped up in his own delusions to listen.
As for the piano incident, the focus of the episode was actually about the KND being injured and replaced by hamsters. The piano incident was the pretext for their injuries to occur, so we weren't given the full context since it was a Cold Open. Given how KND villains tend to behave, in context the pianos were probably part of some villain's scheme to enslave children with piano lessons, or something along those lines. Numbuh Four also confirmed that "they make you play the same song over and over again!"
That's how music lessons WORK. Students practice the same song over and over until they get it right. It's a perfectly normal and natural way to acquire a skill. Also, Numbuh Four seems to have picked up some musical knowledge, he hums "Pop Goes The Weasel" in the piano episode and describes that song as torture, but in the bullfighting episode he is cheerfully humming the Toreador theme from Carmen, not something you would expect the culturally averse Book Dumb character to know.
It's pretty clear in the episode that the mission was called by Numbuh Four, who was personally going to suffer from the piano being delivered(from his point of view). Even then, the Music Teacher sounds like (s)he could be an actual KND villain, so I'd probably go with the above answer that his music lessons were beyond what we would call typical, whereas Numbuh Four has been seen to take an interest in something because he wanted to take an interest, not because he was forced to.
Designated Villain: Done deliberately with Professor XXXL, who in all his appearances is just trying to create the perfect snowcone, but the KND always ruin his plans thinking he is doing something evil.
Numbuh Three×Four were this in the first season thanks to fan reaction to "Operation: O.F.F.I.C.E.". The escalating Ship Tease in the subsequent seasons upgraded them into Official Couple status. Even the last episode revealed that they'd be husband and wife.
Numbuh 65.3×Lizzie: They're both glasses-wearing geeks. In "Operation: L.E.A.D.E.R.", Lizzie pretends to flirt with him and says she likes his pocket protector, making 65.3 blush. In "Operation: G.I.R.L.F.R.I.E.N.D.", 65.3 tells Nigel to say "hi" to Lizzie for him.
The premise of the show is about a war between the kids and the adults, which is odd (and a little depressing), considering that Cartoon Network has the [adult swim] line-up and a lot of kids' cartoons loaded with adult jokes — both clean and risqué (this show being one of them, though it wasn't to an insane degree, like with Time Squad, Johnny Bravo, and others before and after it). Looks like the adults won in the end.
"Operation: G.R.O.W.N.-U.P." has Father and a grown-up Numbuh One facing off, and, after Father comments on how useless the DCFDTL are, Numbuh One responds with "that must run in the family". And then, in the same scene, Numbuh One says that "his Grandma can throw better than [Father]." Which, considering Father is Nigel's uncle... yeah.
The first comment doubles as Hilarious in Hindsight, considering that Grandfather actually does find Father to be useless.
In the Kenny and the Chimp pilot, Kenny got... Swine Flu.
"Operation F.U.T.U.R.E." is a Terminator parody, where girly girls take over the world from an all girls school and the last rowdy boys are fighting a losing battle against them. Many academic studies show that in Real Life the girls are increasingly taking over, due to being generally better students and that today's rowdy boys are going to be tomorrow's losers.
Tiger Moms — the battle against adult tyranny is as important as ever!
Growing the Beard: The Season 1 Finale established continuity for the series, as well as the introduction of Father and the fact that the KND is not just the five featured on the show, but rather many different sectors.
Harsher in Hindsight: In Kenny and the Chimp, one of the joke diseases that Kenny get is Swine Flu. And then in 2009, it came back (swine flu was around in the mid-1970s).
One episode where "chicken pox" takes the form of literal squirming chicken heads sticking out of one's skin.
A villain named the Common Cold attacks by siphoning mucus out of his nose. Especially lovely when he starts to ready a really big attack. Probably why he was Demoted to Extra after his first 2 appearances.
In "Operation: P.I.N.K.E.Y.E.", being hit with a water balloon filled with pinkeye mucus would cause crusty red stuff to grow all over you. The episode's villain turns out to be using this crusty stuff as the crumbs in crumb cake. It gets even worse at the end of the episode: "So if the pinkeye was the crust, then what was the filling?" (achoo!)
Even worse? In two later episodes, Numbuh Two still eats the stuff and he comments about it as well!
Is no one going to mention "Operation: V.I.R.U.S.", where there is a close-up of Cree's gigantic and disgusting pimple?
A few of The Movie's Senior Citizombies have saggingGag Boobs.This included the citizombified versions of Numbuh Three and Five.
The Delightful Children are terrifying when you know who they really are, and what happened to them. They used to be Sector Z, until they were captured by Father himself, and put them through such rigorous torture (or as he puts it, Delightfulization) to the point that's it's quite literally IMPOSSIBLE to turn them back (any attempts are just temporary). To think that such honorable members of the KND have been turned into such monsters is not only scary, but you have to think of what kind of treatment Father put a couple of ten year olds to, not to mention that the torture was so intense that they work as one being.
Heinrich, Numbuh Five's nemesis, trying to dunk a live rabbit in molten chocolate. Terrifying.
Even worse when he soon after tries to dunk Abigail herself into the molten chocolate.
Heinrich used to WORK WITH Abigail before!
From Bad to Worse! Even worse than before! Heinrich fell into the chocolate and became an eldritch abomination!
Grandfather, Father was originally scary, Grandfather takes it to another level. Sure, he's got some humor, but overall, he's just scary. To elaborate, he's capable of turning anyone he touches into a citizombie...which naturally have the same ability as well. He's even capable of zombifying the world in a sense, turning it into a colorless world with tapioca factories for KND treehouses.
The Pirates of the Caribbean parody episode. Somehow, turning into licorice skeletons seems scarier than turning into regular ol' skeletons. And that's not mentioning what happens to them when the sun comes up...
The delightfulization process. Can you imagine being permanently brainwashed into being evil?! There's also the fact that you're probably alive and aware throughout the entire ordeal and maybe even afterwards...
Numbuh Three when she's pissed. She's just plain scary, especially her appearance. And her anger gives a whole new meaning to Beware the Nice Ones.
Ships That Pass In The Night: Numbuh 86 and Numbuh 60 is a rather popular ship, despite the fact that they never really had any onscreen interaction. (The closest was Operation: T.R.A.I.N.I.N.G., when Arctic Base got a distress signal from Moonbase. And even that turned out to be just Father disguised as Numbuh 86.)
They Wasted a Perfectly Good Plot: The KND "Splinter Cell" arc foreshadowed an internal civil war in the KND between the extremist Splinter Cell that believes all adults must be destroyed vs. the more moderate faction that believes only the evil adults should be stopped. It is also revealed that the Splinter Cell had been kidnapping KND scientists for unknown reasons. However, near the end of the series, it turns out that the Splinter Cell is just an front for the KND scientists, who fake their kidnappings so they have more free time to watch their favorite science fiction shows as well as manage "the Galactic Kids Next Door", the workings of which were left very vague in the end.
There's also the matter of the 1/362 Ship Tease. Despite Operation: I.T. showing how close they were and Operation: G.I.R.L.F.R.I.E.N.D. all-but-confirming that Rachel has feeling for Nigel beyond friendship, it was never resolved. The trope comes in with after Lizzie dumped Nigel, which looked like it was foreshadowing that he would end up with Rachel. You can imagine how many fanfics have centered around them getting together.
In the main team alone; 3+4 didn't so much have subtext as it was just plain text from the start, 2+5 was a slightly more debatable case, and 1+5 had some good chemistry. But let's not get into the web that the fans (and creators) have spun.
Unfortunate Implications: In Op CARAMEL, we find out that bitter villain Heinrich Von Marzipan is really a girl magically turned into an evil boy. After he's returned to being Henrietta Non Marzipan, she instantly stops being a villain. So apparently the cure to all male evilness is to become female.
Lizzie brainwashes Numbah One. He breaks out of it... and keeps dating her. Easily Forgiven much?
Villain Decay: Father started out as... well, pretty much Satan. Scary, always cloaked in darkness, and biblical in his wrath- and of course, his voice is Maurice LaMarche at his scenery-chewing best. Contrast this with the middle-aged dork he became later on.
Mega Mom and Destructo Dad were considered an extreme threat to the KND as a whole in their first appearance. It was also implied they took out the entire Kids Next Door (Not sure about Moonbase Operatives) with the exception of their son, Numbuh 274. Besides of the fact that often attend all of the Villain gatherings, their connection to Chad is never mentioned again, even after he becomes a Double Agent. Also, they are never seen using their powers in future appearances.