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.
Angst? What Angst?: Bradford likes his mutation into Rahzar despite it being much more horrific looking than his Dog Pound mutation, since it allowed him to regain some of his ninja speed. Subverted in "The Lonely Mutation of Baxter Stockman", where it is shown he still wants to be human again.
Because a filler episode comes afterwards, April's reaction to learning she's a human-Kraang mutant comes off as this.
Arguably, Ice Cream Kitty. She doesn't seem bothered in the slightest about being transformed into a living dessert.
Anvilicious: The moral about not letting insults get to you, to the point where Raph is blatantly portrayed as putting his "insult ignoring training" to "good use".
Awesome Art: The show is one of Nick's better animated CGI cartoons. The acute eye will notice subtle "blink and you'll miss it" moments about the body language of the characters and facial expressions that really help set the mood for certain scenes. The occasional 2D elements that get thrown in every so often make the show visually fun to look at.
Then there's the fight scenes. They flow well, feel realistic, and can be both fun and exciting to watch. One of the best examples would be Splinter and Shredder's fight at the end if season one.
Base Breaker: Either Michelangelo is funnier than ever, or he's more annoying than ever.
Casey Jones, even before he appeared in the show!
Donatello. You either think his crush on April is cute or Squick.
April's being angry with the Turtles after they accidentally mutated her dad has caused either people to turn against her or explain why she's justified in acting that way. Might calm down after "Target: April O' Neil", where she apologizes to the Turtles for acting like they did it on purpose and apologizing for the way she's acted to them.
Seth Green's voice as Leo in Season 3. Whereas Dominic Catrambone managed to sound almost exactly like Jason Biggs, Seth Green sounds nowhere near it, being borderline baritone. Some don't mind it, others do.
In the least, it's given a reason, in contrast to others cartoons that change voice actors without the in-universe characters noting the change. Shredder damaged his vocal chords to the point of being unrepairable.
Karai's mutation. Some fans hate it due to the fact that it came up simply to keep her and Splinter apart per the status quo. Others hate it due to it being a heavy change to the show's canon since Karai has never been a mutant in any other iteration of the series, with some feeling it's a cheap gimmick of adding yet another mutant to the toyline, not helped by the fact that a figure was indeed announced not long after the change happened. Others are fine with it, as long as they feel it can be eventually cured down the line.
Tiger Claw is either awesome or a Villain Sue depending on who you ask.
The end of "A Foot Too Big" where after Donnie comes to the realization that he's been too much of a Stalker with a Crush towards April and apologizes, saying he'll lay off and then she kisses him on the lips. some fans are crying foul about the mixed messages April's giving by her response, others are happy for their ship, and still others are just pissed off at the writers/execs for clearly trying to drag out the love triangle.
Big Lipped Alligator Moment: A mildly disturbing occurs in Fungus Humongus: After Leo gets sprayed with spore and starts seeing his fear, we're all treated to a random scene of all the mushrooms dancing to the Nutcracker Suite. However, it does break the tension briefly of one of the more disturbing episodes.
The ending shows one of the mushrooms on Mikey's pizza getting up and dancing.
Also the cheese phone from The Manhattan Project.
Slash's ability to perfectly mimic Leonardo's voice and silhouette.
Bizarro Episode: Even by this shows standards, "Pizza Face" is just...out there.
A close second in weirdness is "A Foot Too Big". To-wit: The Turtles find Bigfoot, Bigfoot falls in love with Donnie and a psycho hunter who talks to his mothers shrunken head is after them.
People either love the new personalities for the Turtles, or think it's crap.
Broadly speaking, their personalities still line up with both previous shows' theme songs. The biggest change is Donatello having a crush on young teen April (and Raphael is somewhere between the 1987 and 2003 shows on the anger scale).
The quality of this version of the turtles is arguably the most divided when it comes to the fans reactions of the whole franchise.
The reworked designs of the Turtles seem to be something of a point of contention among a few fans.
Fans are also divided over whether or not it is a good thing that Spike became Slash.
Season 2 seems to have divided fans. There are those who see it as Growing the Beard, and there are others that feel that while the season has some good episodes, it also has a few pacing issues, such as resolving plots too quickly, having perhaps too many filler episodes, some of which are very goofy and at odds with the slightly darker tone that the season was going for in its first few episodes.
While fans rejoiced that Bebop and Rocksteady are finally appearing, they are heavily divided over 2 things. The first is their drastically different character designs◊. The second thing is the fact that they're completely different characters from the other versions. Basically, instead of the loveable idiots from the 80's and comics universes, Bebop is an eccentric hi-tech thief who speaks in slang, and Rocksteady is a Russian arms dealer, who is also a collector of antique military weapons, and is after Shredder's helmet.
The reveal in "The Invasion, Part 1" that Irma was really a Kraang spy. Some found the twist funny, while others hated it because it came completely out of nowhere with no foreshadowing except for one part in the previous episode. It requires a lot of suspension of disbelief to buy that she was a robot the entire time and a lot of die hard Irma fans are clinging on to the theory that the real Irma was replaced by a robot until Word of God proves otherwise.
The fact that the Kraang as a whole have seem to have replaced long time Big Bad Shredder as the main villain of the entire series.
Splinter+The empty broken mutagen canister Mikey called "mom."
Leo+Karai's throwing knife.
Character Rerailment: One of the most consistent problems that were had with Turtles Forever was the Character Exaggeration the 1987 Turtles went through, save particular quirks, where they were portrayed as being a bit too silly and too similar. Their brief appearances in "The Manhattan Project" dial back the silliness and uses the characterizations more common to the 1987 series.
Continuity Lock-Out: The first season alone demonstrates this, due to the fact that episodes build on each other, lack of Previously On segments (though the opener may explain), and not really explaining things as well as they could for viewers who have missed an episode or are just starting (though Nick.com is kind enough to leave episodes up for a few weeks just in case viewers missed them).
Crack Ship: Donatello/April shippers and Casey/April shippers have been at each other's throats since the first episode. Those who have been annoyed by their behavior have started shipping Casey/Donatello to troll them for their irrational behavior.
April/Raphael to a lesser, yet somehow still understandable, degree.
Damsel Scrappy: April seems to keep on becoming the Damsel in Distress despite her alleged kunoichi training, and by mid-Season 2 it's starting to wear thin on some viewers. The episode "Mutants and Monsters" even pokes fun at the trend by making an illusionary April play the part of a Princess Classic needing to be saved.
Die for Our Ship: Casey Jones has been hit with this by many April/Donatello shippers.
This reaches into Fandumb territory as some April/Donnie shippers have declared the show of losing "quality" and have declared they will not watch it anymore... simply for the ship teasing of the (almost always canon) Casey/April ship.
Dude, Not Funny!: The turtles laughing at Donnie and making puns at his expense after getting beaten up by Dr. Rockwell in Monkey Brains.
The turtles accidentally almost killing Splinter with Donnie's naginata in I, Monster. This type of close call situation can be funny if the character is perfectly healthy, not when the character is currently being subject to mind rape and subsequent mental pain.
Ear Worm: The theme song; seems to be a tradition in the TMNT franchise.
This show's version of Karai has developed a following. Her loyalty to the Shredder/Foot is downplayed, her snarks are hilarious, and her Ship Tease with Leo helps (or doesn't).
Xever/Fishface is slightly more popular than his partner Chris Bradford/Dogpound, due in part to having a more visually appealing, unique fighting style. It probably helps that the audience learns far more about him than they've learned about his partner.
Spike and Murakami, thanks to being a funny background critter and a Cool Old Guy respectively..
Despite starting out as The Scrappy (mostly due to shipping), Casey has since become a fan favorite.
Out of the four turtles Raphael seems to be the favorite, possibly due to his Hidden Depths and being a Fountain of Memes. Second to Raphael would possibly have to be Leonardo, who not only has been Rescued from the Scrappy Heap compared to his previous incarnations, but is generally considered to be one of the least obnoxious turtles with his love of cartoons and focus on learning to be a leader being positive highlights.
Starting out as divided when people found out about the character Slash is now very well liked due to being portrayed as a cunning, genuine threat who provides interesting tension with Raphael.
Ice Cream Kitty seems to have developed a following.
Tiger Claw has some of this due to being a Badass bounty hunter who puts up a brutal fight with the turtles.
In "New Friend, Old Enemy", Splinter teaches Leonardo that victory is more important than fairness. Granted, he's talking about fighting (potentially) to the death, but perhaps they should have clarified that to the kids watching.
New Girl in Town has Splinter deliver a message to Leonardo that leadership is a hard, thankless task, and he should not think otherwise.
Fan-Preferred Couple: April/Raph ever since the creators released old concept art of the the two dancing together. (Raph noticeably had hearts in his eyes and the writers even went so far as to confirm that they were originally going to be love interests.) Basically due to Casey being a bit of a perv and Donnie being down right creepy at times support for this pairing has skyrocketed.
April/Leo also has quite the following. This is mostly due to April having far more interaction with him than the other turtles sans Donnie.
Foe Yay: Leo and Karai. April and Raphael clearly do not approve of this development.
April and Karai have some too. "Miss me, princess?"
Fountain of Memes: Suprisingly enough this version of Raphael has been the source of most of the show's memes.
Genius Bonus/Fridge Brilliance: In Episode 13, the Rat King wears an outfit nearly identical to the ones worn by medieval Plague doctors (long black coat and black wide-brimmed hat). The Bubonic plague was spread (partly) because of fleas being carried by rats.
In Leatherhead's first episode,he grabs one of the turtles and starts spinning. A grab-and-roll is a tactic used by real life alligators to take down their prey.
Growing the Beard: The tone of the show became more focused and consistent mid-first season, starting with "The Gauntlet", the arcs that were previously only hinted at or set up became more prominent, and mostly dropped the Monster of the Week format. Later in Season 1, Karai's introduction upshot an interesting dynamic with Leonardo, some new and interesting setpieces were introduced, and the fight scenes and animation becoming far more consistent and fluid. Season 2 seems to be helping it along as well, due to heading into a somewhat darker direction.
Harsher in Hindsight: At the beginning of Rise of the Turtles Part 2, Mikey wants to drink some mutagen, but Don points out that it may turn him into a pile of goo. Fast forward to The Pulverizer Returns....
Also fast forward to Mikey Gets Shellacne where we see what happens when Mikey does use more mutagen on himself.
Any moment between Raphael and his pet turtle, Spike, become this after Spike mutates into Slash.
Any moment that Irma is on-screen becomes this after the reveal that Irma is a Kraang in disguise who became April's friend in order to get April to lead her to the turtles. You can see how everything this character says is a subtle manipulation to earn April's trust in her, and sometimes to erode April's trust in others, such as Casey. Every little quirk is a foreshadowing of what this character really is, such as her encyclopedic knowledge of the behaviors of rats, an innocent enough interest for a Perky Goth, but also something valuable when tracking Master Splinter. Or the gluten-free diet a Kraangbot mentions in passing, when Irma is the only prominent "human" character to stick to the same diet.
Hilarious in Hindsight: Donatello/April and Leonardo/Karai were both ships that only received mere teasing in the 2003 series rather than anything serious. This series takes those teases and makes them officially acknowledged in-universe.
Donatello and Casey's friendly rivalry and name calling in the 1990 film, all of a sudden becomes a lot funnier now that the two have received Die for Our Ship between those who pair them with April in this series.
Ho Yay: Mikey and Leatherhead in "It Came From The Depths".
Irma seems to be just as jealous of Casey and possessive of April as Donnie is.
Becomes a little squicky when you realize that Irma is a little less Goth and a little more... Gottfried.
Idiot Plot: "Mikey Gets Shellacne" is started by Michelangelo using prototype Retro-Mutagen on himself, which will have the side effect of making him explode due to the incompleteness of it, because he completely ignored the labels telling him not to use it. To a more minor extent, the other characters ignore Mikey thanks to him messing up when he says that Baxter is probably at his lab, which, considering Baxter's personality, actually wouldn't be very far-off. The entire episode ends up resulting in a far more powerful Chris Bradford, who mutated into Rahzar thanks to it.
Jerkass Woobie: Raph in Cockroach Terminator. Raph shows that he's clearly afraid of regular cockroaches, but then he has to put up with a mutant cockroach and the other turtles are absolutely no help since they either make fun of him or don't give him any sympathy.
Happens to him again in season 2 when Spike becomes Slash, hurts his brothers, and has to fight him. The ending where Raph is looking at a chewed up leaf really sends the message that his best friend is gone and has turned into a monster.
As of the season finale, Karai now qualifies, as she is actually Splinter's daughter and has been lied to her whole life by Shredder, and she still thinks that Splinter's the one who killed her mother and wants vengeance.
The ending of Wormquake/The Manhattan Project shows her eavesdropping on the heroes, where April is in disbelief that Karai's actually Splinter's daughter. Splinter states that Karai might believe the truth one day, but its ultimately her decision.
It gets worse for the poor girl once she DOES believe the truth in "The Wrath of Tiger Claw" and "Vengeance is Mine".
Arguably Spike, while he did try and murder the turtles he genuinely believed he was helping Raph... it's kinda hard NOT to feel bad for him when Raph calls him a monster.
Baxter Stockman in his more pathetic moments can be this. And it gets worse after his mutation, as it drove him over the edge, and all he wants is to be normal again.
That which is the way the Kraang speak has become mutated by that which is known as the Internet into what is known as a meme.
The term "sadorable" has become extremely popular on Deviantart.
A certain preview for the two part finale for season one shows the turtles and April having a little dance party. All three dances shown has become a bit memetic. So you know it shows Leo shuffling. Donnie and April, of all dances they could of done, doing the funky chicken. Then Raph's dance, which might be the most memetic of all.
"[Insert character name here] confirmed for season 2!" (To the point where fans will search for the most obscure characters possible to keep the joke going.) It'll probably die down once season 2 actually comes along though.explanation This originated from the writers announcing that Casey Jones would appear in season two before season one even aired.
Moral Event Horizon: Shredder killed Hamato Yoshi's (this was before he was mutated and started calling himself Splinter) wife, burned his house down and kidnapped Yoshi's daughter Miwa to raise as his own, naming her Karai (meaning "weapon"), blaming Yoshi for murdering her mother, and making Karai hate her biological father. Why did he do all this? Essentially because he was jealous of Yoshi.
What makes this even worse is that he's outright shown he has not compassion at all for Karai. She's simply a means to an end. Even the fact she genuinely loves him means nothing so long as he can use her to hurt Splinter. The big crossing in this regard comes when he responds to her asking if Splinter is her real father by saying yes and having her locked up. No show of sorrow or upset. No feelings of remorse. He just basically tells her 'yes, I used you as a living tool of revenge and discards her.
Him blaming Splinter for Karai's mutation probably counts as a better example, but for different reasons. Her mutation was more or less entirely his own fault, with Splinter having nothing to do with it. In a way, Shredder actually caring about Karai makes it worse because instead of going My God, What Have I Done?, he uses it as more fuel for his hunger for revenge instead of accepting responsibility for his actions.
Spike at first came off as a Well-Intentioned Extremist because Raph was mad at his brothers. However, he crossed it when he not only ignored Raph's pleas to not hurt the other turtles, but went after him too.
While his first appearance wasn't exactly not crossing it, The Rat King crossed it in "I Monster" where he sicked a mind controlled Splinter to kill the turtles.
Narm: Mikey calling Kirby "Wing Nut" after he's been mutated into a bat and the turtles are trying to trap him. Which is lampshaded by Donnie.
Donnie: You are not giving Mr. O'Neil a monster name!
Nausea Fuel: Mikey's shell acne throughout the whole episode of Mikey Gets Shell Acne.
Baxter Fly, being part fly, needs to vomit on food before eating it. He also eats garbage.
Some of the toppings Mikey used for his home made pizza in Pizza Face has a clump of dirt with worms (and it looks like he got it from a trash bin), a rat carcass, and an eyeball.
Nightmare Fuel: Fungus Humongus was a disturbing episode and probably viewed with discretion if you're afraid of rats or cockroaches as the visuals (especially with the roaches) can be frightful. Also deals with different fears such as phobias of things or personal dreads of failure as a leader or rejection out of anxiety and guilt.
Nightmare Retardant: While there is a certain paranoia factor to the Kraang, the "place" where they speak creates a difficult "place" with the "place" where they are taken in the serious "place". They can however be surprisingly frightening when they drop that.
Although this can be made sinister when they want it to: "The turtles must be eliminated from this place, and all other places."
As horrifying as the Pulverizer's mutation into Mutagen Man is, there's just something hard to take seriously about his high-pitched voice when he screams "It burns!"
Kraang Primes's voice was this for many people. Perhaps inevitable, considering that they were voiced by Roseanne Barr, who is known as a comedian first and the casting choice was considered pretty strange to begin with.
Much of the scare factor from "Fungus Humongous" can be ruined due to the fact that, aside from Leonardo, April, and maybe Casey, if you're a musophobic, the other Turtles' fears and hallucinations are done in a manner too over-the-top or silly to be taken seriously, with Donatello possibly being the worst offender, as his hallucinations of April rejecting him is done in the same Animesque, exaggerated style as the rest of the series.
Leonardo tended to get a lot of reactions ranging from apathy to dislike from fans in previous incarnations due to being, what they felt, to be a rather boring character. This incarnation gets a fair amount of like due to the show making an effort to showcase him as still learning to be a leader, as he has just started, and the addition of other characteristics (his love of Space Heroes and disobeying Splinter a bit more frequently).
One can now argue Raph has been rescued from the heap after "Operation: Breakout" as he took several levels of kindness within the episode. A notable example is his behavior towards Mikey in the episode. In the beginning he intentionally does something he knows Mikey can't get enough (albiet at the expense of Donnie), his once an episode Dope Slap came off as a warning for Mikey to mind his manners (Mikey basically just called Leo stupid, which visibly made Leo angry) and near the end he laughs at one of Mikey's jokes while patting his head. It helps that he has similar behavior towards Donnie in the episode. When he finds out that, yes Donnie still has a crush on April he recommends Donnie not involve him, both as something he obviously thought was helpful advice (since he just embarrassed Donnie) and because he was clearly a little angry Donnie was, in a sense, using him. This has unintended consequences as Donnie leaves without the others on a resque mission for April's dad. He obviously felt bad when he realized this and in the end retold the episodes events in a way that made Donnie look like the big hero of the day which resulted in Donnie getting a hug from April. It probably helped even more that he was called out by Leo for his negative behavior and then, as stated above, made up for it by the end of the episode.
Michelangelo is frequently derided for being even dumber than his previous incarnations, but has earned some likability for having occasional moments of competence in episodes like "New Friend, Old Enemy" and "Parasitica".
Donatello when he finally realized that he should stop pursuing April in season 3. Sure, it's implied that he still has feeling for her, but that was mostly April's fault this time.
Romantic Plot Tumor: Almost every episode featuring April in a big role is going to heavily feature Donatello's obsessive crush on her, and it's gotten to the point where many are tired of it, especially now that Casey Jones is in the picture and making it a love triangle.
Michelangelo gets this from one side of the Broken Base, due to feeling the show downplayed his Hidden Depths in favor of making a bigger deal on his humorous side, which results in him being grating due to him missing the point of usually very high stakes consequences.
The hate seems to be magnified a bit with him constantly whining about the Rat King's name in the latter's debut episode. Aside from the fact that it seems like a Take That to a classic character with an established history and fanbase, Michelangelo seems to think he has dibs on how villains should name themselves and would not shut up about it for the whole episode.
Raphael, due to being a bigger jerk in this series than he's usually depcited, has his fair share of people who dislike him. This has cooled off a bit though, thanks to getting Hidden Depths and toning down his behavior in later episodes.
Donnie has landed himself in this by the end of season one. His crush on April was viewed by fans as cute and even endearing during early season one, then suddenly it stopped being referenced to for a few episodes, then it came back but extremely flanderized. More focus starts getting put on it and we get to see Donnie makes less than rational choices because of it. Then there was Operation: Breakout. Raph tells Donnie to leave him out of his attempts at impress April. (Raph clearly didn't like being used like that and was offering some advise he seemed to think was helpful.) So what does Donnie do? He basically goes on a VERY dangerous rescue mission, by himself, without telling anybody, to get April's dad away from the Kranng. Not only that but he almost gets himself and Mr.O'Neal killed multiple times. All to impress April.
Outside of the turtles are the Kraang, as they get this mostly for the fact that once they start talking, they're almost never going to shut up. As time has gone on, the show seems to have tried to phase out the humor aspects, possibly saving them a little.
The Pulverizer is quite disliked for being an obnoxious fanboy whose levels of stupidity range from uncanny to mind blowing.
Spiderbytez, an intentional case, for being a crude, realistically obnoxious person before his mutation, and only got worse upon it. Does(n't) help that grumpy neighbors like him exist and are one IRL.
Seasonal Rot: Many fans of the first season have expressed dislike or disappointment in season two. The cause of this occurred behind the scenes, as the writers from the first season were all replaced with brand new-writers unfamiliar with the show for season two. As a result, it took the new writers a good while to really find their groove, and the results are noticeable. Pacing Problems began showing up with some plot elements being resolved too quickly while others wound up taking forever, the turtles, particularly Mikey, all went through some Flanderization, several new characters were introduced but many of them were underutilized, and many controversial changes began happening which left people divided all over the place. That said, the final few episodes do show noticeable improvement and the season finale was particularly well-received, with the exception of the Irma twist.
Special Effect Failure: While the show's CGI is generally fairly good, sometimes an acute eye will spot that there's something off about the skyline or the distant buildings.
The Invasion two-parter had it snowing, along with generally well-detailed buildings and environments to reflect this in Part 1 and for most of Part 2, but by night-time, the snow disappeared completely.
Shipping: Donnie and April have been getting this since the first episode. Also, people started shipping Leo with Karai as soon as the description for "New Girl in Town" was released (a legacy of the 2003 show).
Casey didn't appear until Season 2, but he got Die for Our Ship from Donnie X April shippers long before his debut.
Some fans got so tired of the drama surrounding April, Donnie, and Casey that they disregarded the love triangle completely and began "bro-shipping" Casey and Raph instead.
Some Anvils Need to Be Dropped: It seems the show has a minor theme about not allowing friendships to be ruined. Splinter and Shredder were once the best of friends until they competed for Tang Shen's affections. Saki went as far as to provoke Splinter, and instead of trying to mend their friendship, the two allowed their rivalry to become hatred, resulting in Splinter losing everything he loved. Then there's the incident with April turning her back on the turtles after they accidentally mutate her father into a giant bat. Casey tells her about how he unintentionally allowed his best friend Nick to hate him after accidentally hurting him, which convinces her not to allow an accident to turn her back on her friends. Had Splinter and Shredder did what April had done and try remain friends after everything, Splinter could have lived a happier life while Shredder could have moved on.
In-Universe, the Turtles reaction to the sea monster guarding the Kraang fortress falling in love with their submarine.
Depending on whether or not you believe the mutagen made the turtles related to Splinter by blood the fact that Karai is infact Splinter's daughter may have very well caused this reaction in some fans. Especially since Leo had/has a crush on her and Raph admitted she's hot "in the evil way".
The people who were guessing that this version of Karai and Splinter are related since before the show even aired (due to promotional material) found the Leo+Karai ship gross, especially if they're in the party that believe the Turtles and Splinter are related thanks to the mutagen. The confirmation that yes, Karai is Splinter's daughter has caused several people to abandon the Leo+Karai ship.
Some fans find any mutant+human shipping to be a bit...odd.
Concept art notes◊ on the Mousers reveal that their "gut" carries 5 Rat Carcasses. Ew.
Wormquake/The Manhattan Project reveals that mutagen comes from alien worms and the mutagen has to be milked from them.
At the end of "Target: April O' Neil" Mikey takes out a slice of pizza with bugs on it and eats it.
The pizza that Mikey bakes in "Pizza Face". Just look at it.◊ Undestandably everyone was grossed out when Mikey actually ates it.
Stop Helping Me!: The turtle's basic reaction to anything "The Pulverizer"/Timothy does to try and be more like them or be useful to them.
Believe it or not, the most controversial change is the addition of a third toe on the Turtles. Fans exploded.
April being de-aged to a teenager hasn't been taken very well. And as of "Monkey Brains," people are complaining about her "empathic" ability and Splinter's decision to train her in ninjutsu because of it.
Neither has the replacement of "Cowabunga!" with "Booyakasha!"
Though the decision to make her Hamato Yoshi's biological daughter is something that threw fans off. Especially since they saw that plot-twist coming a mile away.
Within this particular version, the second season's Darker and Edgier turn, in which April's father becomes a mutant, and as a result now hates the Turtles for causing it to happen, despite the fact that it was an accident.
The revelation that April is a half-Kraang mutant is already starting to turn some people off.
Dominic Catrambone replacing Jason Biggs as Leonardo has sparked a lot of complaints.
Seth Green being confirmed to take over beginning in season 3 has provoked a similar reaction.
April also qualifies. She's being trained as a kunoichi, is now confirmed to be half-Kraang and half-human, is immune to Mutagen and has empathy/telepathy powers. Although this makes her cliche to some, the writers could have actually done something with it. Instead, everything is completely tossed over. Despite her training, she's kidnapped or in danger in almost every episode. The half-Kraang, half-human idea is hardly ever mentioned again. Her immunity is understandable, but her powers make her the source of last-minute solutions to problem. As of the love triangle between her, Casey, and Donny, she's little more than a plot device only there to be kidnapped and ignore Donny's affections.
Irma makes her first appearance since the 80's cartoons, to the delight of fans. There is teasing over when she will finally meet the turtles and how her interactions with them and Splinter will be like, and one episode establishes that she may have an unknown, hidden power, which would have made this Irma more fleshed out than the boy-crazy best friend from the original cartoon. Instead, Irma was never real to begin with and was actually a Kraang all along, used solely to find the turtles' lair and quickly disposed of by Splinter, which rather feels like the writers just gave the middle finger to long-time fans who wanted Irma to be among the supporting cast.
Xever has interesting chemistry with Raphael, a unique fighting style among all the ninjitsu-using characters in the series, and is a surprisingly threatening and dangerous planner, which makes Season 2's treatment (appearing less often and being played more for humor) of the character disappointing.
While stylized like everything else in the show, the Kraang's disguises are droll businessmen whose eyes and mouths scarcely move, as if the Kraang are barely trying with their disguises, creating a rather odd and creepy effect.
Then there's Mrs. Campbell, who is also a Kraang creation, and only gets creepier during her fight with the turtles.
There's something creepy about the realistic lips that the mutated Pulverizer aka Mutagen Man has.
The Untwist: Many fans saw the first season finale's twist about Splinter's daughter coming a mile away. The fact that Hamato Yoshi never had a daughter in other continuities and that in continuities where she was Shredder's daughter, Karai was always explicitly referred to as his adopted daughter is what made it easy to predict.
Villain Sue: Tiger Claw. Beyond being absolutely pumped up by the Shredder as a Badass and automatically being promoted as his Number Two on his first day over all of his other minions, including Rahzar and Karai, he often manages to defeat all the turtles (and anyone else fighting with them at the same time) single-handedly. Plus he's one of the few villians on the show with little to no backstory, so to speak. However, it seems the writers are making him more fallible as he's taken more losses (especially in the finale), so time will tell when he's out of here.
What an Idiot: Episode 6. A Kraang takes control of Donnie's robot and the guys futilely attack it, claiming it has to have some sort of weak spot...completely ignoring the Kraang that's just sitting on its head right out there in the open.
The season 2 premiere has the turtles (except for Donnie) playing dodgeball with the Kraang communication orb. Which Donnie calls them out on. Nothing new for Mikey and Raph, but Leo should've known better. Sure nothing bad came of it but still.
A better example would be Mikey telling April that it was their fault that her father became mutated, causing her to hate them
Mikey holds the ball even harder in the "Shellacne" episode, since in order to make himself cooler he exposed himself to dangerous experimental retro-mutagen, missing the fact that Donatello put several warnings of how deadly it was on the can since he only looked at the front.
A non-lethal example but still worth a facepalm. In Pizza Face, Mikey tries to take pictures of all the people that are being mind controlled by Antonio. When he's done and checks the camera, all of the pictures consist of his eye, and he thinks the camera's broken, not realizing he was pointing the camera in the wrong direction.
Anton Zeck is out running in the middle of the street while invisible where no drivers can see him. No wonder the Shellraiser hit him.