Season 5 of Aqua Teen Hunger Force. Largely after the writers were given free rein contentwise, meaning we got a ton of overtly gross-out storylines with rotting corpses, bestiality, and Master Shake microwaving a cat.
Season 4 (and so far, the final season) of The Boondocks is considered a severe drop in quality from the first 3 seasons. This is largely due to the fact that Aaron McGruder no longer works on the show, several characters's voice actors leaving the show (like Ed the 3rd, Gin Rummy, and Thugnificent) causing them to be omitted entirely, Ruckus getting flanderized to the point where he isn't funny anymore, the increased focus on Grandad and the season having an overarching plot (The Freemans being in debt due to Grandad falling for an obvious internet scam) rather than standalone stories. The season aired its episodes out of production order so it seemed to normal viewers that the arc was never resolved, when it actually was resolved in "Early Bird Special", the 4th episode of the season aired. The season's final episode is about Riley who first pisses off the LGBT community, and then the mentally disabled. It and so far the series ultimately ends with a photo of Riley running from an angry mob of mentally disabled people that he personally pissed off. Fans have declared Fanon Discontinuity on this season because of the rot. Even Adult Swim seems to have picked up on this and hardly, if ever, reruns episodes from the season.
While Seasons 3 and 4 created a lot of Broken Base, Season 5 is pretty much universally reviled by the fandom. This is due to:
The retconing of important continuity points. A major one being that Daphne wasn't actually dead but cursed into becoming a spirit.
Characters are flanderized and acting like children despite having already at this point graduated Alfea College.Stella and Tecna get the worst of this, basically undoing all the Character Development they've gotten over the series and then some.
Unnecessary, jarring, and downright ugly shifts to computer graphics. Sure, the movies weren't ever Pixar quality, but it least didn't look like they came out of the Uncanny Valley.
Some fans weren't very pleased with season 3 of Avatar The Last Airbender due to the large amount of filler episodes. Many fan-favorites were sidelined such as Toph, Iroh, and Ty Lee while others were somewhat flanderized. Fans felt like this season wasted a lot of opportunities such as Iroh and Ozai's past, the return of Ursa, more information on Aang's old friend Kuzon, and a possible reappearance of Koh the Face Stealer. Though fans were pleased that we were given backstories for both Avatar Roku and Fire Lord Sozin, many find it to be the most blatant example of Writers Cannot Do Math ever and weren't pleased with some of the contradictions and retcons that were caused by it. Lastly, quite a few fans were displeased with Aang's sudden inability to kill despite having done so in the past, and the fact that he knew for months that he would likely have to kill Ozai.
Even fewer fans liked the "Vacation in Europe" side-season, which had cheap animation, had loads of They Just Didn't Care when it came to European culture and history, and barely fit in with the series' continuity.
Season 6 is generally considered the weak point of the 2003 series, due to being much Lighter and Softer than previous seasons, sending the Turtles to the future for no apparent reason, introducing a supporting cast member in the form of April and Casey's Child Prodigy great-grandson, and, oh yeah, the fact that, on the order of the toy company executives, the writers were forced to skip directly from season 4 to season 6 due to the executives feeling season 5 wouldn't sell toys as well. This was particularly painful because season 5 contained the resolution of the show's Myth Arc. And once season 5 did finally come out to much acclaim, the executives' orders seemed even more nonsensical; wouldn't characters like the Acolytes, the true forms of the Foot Mystics and Ninja Tribunal, the Tengu and his demonic minions, and the Turtles' dragon forms have made great toys?
The 7th and final season, Back to the Sewers is virtually hated by everyone. Despite returning the show back to its present day roots, it adopted a new art style that turned off many viewers, and whatever seriousness that was still leftover in the show from Fast Forward was thrown out altogether to make the show even goofier.
The second season of the 2012 Ninja Turtles cartoon has been regarded by many as weaker than the first. Grievances fans have had include all of the writers from the first season being replaced behind the scenes, Jason Biggs leaving three quarters into the season which necessitated a sudden recast for Leonardo, the tone of the season simultaneously going Darker and EdgierandDenser and Wackier making if feel very uneven, several controversial changes to the show's canon occurring such as the revelation that April has Kraang DNA and has always been a half-alien-mutant or Karai becoming a snake mutant, notable pacing problems (for example, at the end of episode 3 Splinter finally relents and reveals to Leonardo that Karai is his biological daughter, but Leo's reaction to it doesn't come up for another 10 episodes), noticeable Flanderization going on for the Turtles, a very much hated Love Triangle between Donatello, April, and Casey that lasted most of the season, the toyetic nature of the show becoming much more blatant with nearly every episode introducing a new Monster of the Week to later be used in the toyline, and many characters being underutilized (such as Slash being introduced as a formidable enemy of the Turtles only to quickly become an ally in his next major appearance or Irma being introduced into the '12 iteration halfway through the season only to later turn out to be a Kraang spy and then destroyed).
Drawn Together, the first half of season 3, when the show became too darkfor its own good and constantly gave Captain Hero screen time (a trend started since the middle of season 2). This combined with an erratic airing schedule resulted in the show dying an early death.
Season 6 is this for many viewers, largely due to the backlash against Parker and Stone retiring Kenny and their plans for Butters being the new Butt Monkey being changed by Comedy Central and fans rescuing Butters from the Scrappy heap. Needless to say, ever since that season, Parker and Stone have openly threatened to quit production of the show (to the point that Parker almost bailed entirely midway through season eight). Fan opinion of it has warmed up a bit after time, but it's still considered one of the weakest "golden age" seasons.
Seasons 15 onwards have gotten this from many fans to the point of where some people actually hate everything from Seasons 5 and after for changing the character's personalities, becoming more political and almost feeling like an entirely different show.
Season 17 is seen as this by many fans due to its short length, bad characterization (especially in regards to characters such as Cartman, Kyle, and Stan), and overly topical (to We're Still Relevant, Dammit levels) plots and humor. The only thing generally liked about it is the "Black Friday" 3-parter, but even that has its open detractors. Season 18 was criticized for the exact same reasons, though the Story Arc and better attempts at continuity got it a little praise.
Code Lyoko fans consider Seasons 3 and 4 (or if not that, just Season 3) to be inferior to the first two story-wise (though far superior stylistically). While Season 2 involved and ended with an exploration into the computer's past and the progression of the major story arcs, the next two seasons shunted that to the side in favor of episodic filler, which was more often than not dedicated to the increasingly-unfunny escapades of the comic relief characters, particularly Jim.
Season 3 of Danny Phantom suffered from this due to Executive Meddling. Nickelodeon ordered the series to be cancelled with the third season, abruptly fired all the original writers after the second season finale, and ordered only thirteen episodes for the third season, as opposed to the previous two having twenty each. This resulted in incredibly inferior writing, boring villains, a rushed plot with a rushed finale, and Danny and Sam falling in love without any development.
Every Total Drama season following the first has been accused of its problems, but only two are almost universally loathed:
The second season, Total Drama Action was received quite poorly by its viewers, half the original fan base of the original Islanddon't watch it at all. Why? Well, for starters they removed almost half of the original cast, including popular characters (at the time) Cody and Noah, completely flanderized the remaining characters, kicked off most of the fan favorites (again, at the time) such as Gwen and Bridgette early, oversaturated Owenagain, and had Chris become such a huge Jerkass that it wasn't even funny anymore. It was considered by many as the black sheep of the franchise prior to All-Stars and it seems that even the creators feel this way since almost all of the plots and characterizations (aside from Gwen and Trent's breakup which was the result of Executive Meddling, Chef's secret alliance with DJ, as well as Geoff's brief derailment into being a Chris clone) have not been referenced since that season ended.
Many say that the 1st half of the fifth season, All-Stars was just as bad as (if not worse than) Action making it the new black sheep of the franchise in the eyes of many. Reasons include wasted developments like Gwen and Courtney's friendship (which is suddenly undone over the course of one episode and rendered most of the season pointless), the return of the shortened season length of only thirteen episodes (this time, pretty much every plotline was dropped after about 2 or 3 episodes) as well as introducing a chopped up version of the theme song that is only about 20 seconds long compared to the full minute long version, the Big Bad being defeated at the push of a button, characters like Gwen and Heather holding the Idiot Ball in order to justify the terms of their eliminationsnote both of which were in episodes written by Terry McGurrin, 3 episodes (one of which being "Sundae Muddy Sundae" which broke up the Gwen and Courtney friendship) were written by first time writer Ed MacDonald who has since become the most hated writer on the show, once again Flanderizing characters such as Lindsay and Sierra, putting center spotlight on Zoey, who is considered by some to be a rather undeveloped character, and the finale featuring next to no focus on any of the eliminated contestants who weren't already part of the show's Spotlight-Stealing Squad due to being trapped in fart-filled balloons and left flying off at the sun with no indication if they survived and Camp Wawanakwa sinking officially renders the entire season pointless. Fan reception proved so negative that Fresh TV came forth to request fan feedback on the season and use said feedback to play a major part in the development of season 6.
Each entry in the Ben 10 franchise has its own seasonal rot:
Ben 10: Ultimate Alien, on the other hand, got the Seasonal Rot going in its second season, with tons of filler episodes, characters further acting like idiots, and a convoluted main arc plot involving the Forever Knights and an Eldritch Abomination that took a hell of a time to come out. It's commonly thought that it got better in the finale, where Vilgax finallygot out of his decay and was portrayed as a Magnificent Bastard, but it's still controversial.
Season 7, due to the amount of episodes focused on Brian's political views, as well as a few awkward episode premises, massive Flanderization of the whole main cast (i.e. Peter's reckless behavior in "Baby Not on Board"), and, of course, a three-minute long Conway Twitty cutaway. Part of the reason for this can be attributed to the 2007-2008 WGA strike that plunged a lot of shows into getting canceled, going on hiatus, or suffering through seasonal rot.
Season 4 was this when it first aired, but is now considered by many fans to be the show's best season (mainly by people who started watching when the show returned).
Season 3 was also this when it originally aired and for a few years, but is now considered to be a part of the show's golden age by many fans.
There was a time between the 7th and 12th seasons where many said that Season 10 was the worst (and to some still is). Episodes that are prime examples of this include "Seahorse Seashell Party" (for as Mr. Enter pointed out, having the worst example of Status Quo Is God), "Screams of Silence: The Story of Brenda Q" (portraying domestic violence totally straight on a show that makes a living out of doing it for laughs both before and since), "The Blind Side" (all blind people are gullible), "Be Careful What You Fish For" (mostly for the subplot with Brian and Stewie), "Tea Peter" (the plot can easily offend anyone who's studied politics and containing a joke saying that Autism is an excuse for kids to act ill-mannered), and "Internal Affairs" (for having Joe and Bonnie nearly break up, thanks to Peter). There's another side of the fandom that does say that it is (or was before season 12) the worst season but only because most of the episodes were completely forgettable.
Some say that season 12 is the current low point for the series. The main reason being Brian getting killed off in "Life of Brian" only to be brought back three weeks later in "Christmas Guy" in a ratings grab so desperate and shallow that it caused several longtime fans to lose whatever respect that they still had for the show by that point. Other episodes that played a big part in why the season is so reviled include "Peter Problems" (for the scene of Peter accidentally eviscerating a live whale while trying to push it back into the ocean using a forklift), "Brian's a Bad Father" (the main plot being Exactly What It Says on the Tin and the subplot ending with an overly-gory scene of Quagmire shooting Peter in the head), "Fresh Heir" (aka "Let's See How Many Unfunny Incest Jokes We Can Force Down Your Throat in 22 Minutes"), and "Herpe, the Love Sore" (Brian intentionally gives Stewie and Chris herpes when he becomes blood brothers with them and the subplot involving Peter, Joe, and Quagmire which has one of the show's worst Unfortunate Implications).
The post-movie episodes of The Powerpuff Girls are considered to have some of the most ridiculous plots, Flanderization of several characters, and dropped most of the action elements to make the show a cheap gag comedy.
The show never was the same from Season 4 onwards.note When the show was Un-Canceled in 2004, due to the box-office gross the feature film adaptation (which was the intended Grand Finale to the show - season 3 ended production back in 2002, until Nickelodeon decided to revive the show) made in addition to the high ratings the show maintained, creator Stephen Hillenburg notably changed his position in the show's production and co creator Derek Drymon left. Once it started back up, the show became much more grotesque compared to the earlier seasons - post-movie seasons have Vulgar Humor, massive flanderization of various cast members (with some being flanderized into ditzes and designated heroes), occasional dark humor, creepyNausea Fuel, and overall less charm.
The general consensus is that Season 4 was when the rot kicked in, and Seasons 5 and 6 were the peak of the rot.note Most notably Season 6, with episodes like "The Splinter", "Choir Boys", "House Fancy" and "Truth or Square". Seasons 7 and 8 are... "mixed" among fans. The good episodes in them are generally an improvement from Season 6, but the bad episodes in them include some of the most reviled episodes of the show.note "A Pal for Gary", "One Coarse Meal", "Demolition Doofus" and "Stuck in the Wringer".
In turn, most people agree Season 9 is a major improvement on Seasons 5-8, with most of the episodes being praised.note The exceptions being "Spongebob You're Fired", "Squid Baby" and "Little Yellow Book".
Even the Pre-Movie era suffered seasonal rot to some people. The latter half of Season 3 gave us only a couple of really good episodes and was the first time the show gave us several bad episodes (like "Party Pooper Pants", "The Great Snail Race" and "Krabby Land"). It's just not that noticeable because Season 4 started out a lot more consistent in it's good episodes.
The Simpsons: Fans of the show have wildly varying opinions on which seasons were good and which ones sucked, but these are considered the worst:
You won't find too many supporters of Season 11.note (For reference, this is the season that killed off Maude Flanders, had Barney give up drinking, made Apu and Manjula the parents of octuplets, and presented whimsically self-referential episodes like "Saddlesore Galactica", "Missionary: Impossible", and "Behind the Laughter".) It was the least grounded and realistic of all Simpsons seasons with more out-there plots and twist endings. It's also rare to see it aired on TV anymore. Season 12 also suffered from this, but to a lesser extent than season 11. Season 13, when Al Jean returned as a showrunner, was a partial return to the days of the less wacky Simpsons. However, these later seasons are often criticised for being when The Simpsons became a Franchise Zombie.
Seasons 9 and 10 get this too, due to "The Principal and the Pauper" (the infamous episode where Principal Skinner is revealed to be a street punk named Armin Tamzarian who took on the identity of a man named Seymour Skinner so he wouldn't have to tell the man's mother that her son was M.I.A.), one of the most hated episodes in the entire series (even the show's creators hated it). Add to that the parade of celebrity cameos and humor getting cruder and more sadistic to compete with South Park, as well as the induction of Mike Scully as showrunner. The season still features a lot of classic episodes, especially the holdovers from the Season 7-8 run, but the ratio of bad to good spikes dramatically and never really gets back in balance. When fan reviewer Mike Amato covered Season 9, it was the first one in which he was able to name five episodes he hated.
Futurama is considered by many to be going through this ever since it made the Channel Hop to Comedy Central. This is largely because of all the Anvilicious episodes that basically turned the show into a cleaner South Park, lampooning current issues with ham-fisted attempts to show the writer's side of the argument (the episode "Proposition Infinity," an obvious Expy for gay rights, comes to mind).
More criticisms include the heavy Flanderization of several characters and a slight increase in Nausea Fuel that still existed but wasn't anywhere near as noticeable in the Fox episodes.
Futurama also has an in-universe example in the form of Everybody LovesHypnotoad, which has been going downhill since its third season. note This is according to someone who's immune, or at least highly resistant to, the Hypnotoad's brainwashing thanks to being his own grandfather.
Teen Titans fans generally consider season 3 to be this, since they changed the Big Bad from the awesomely creepy Slade, who was Robin's archrival but still had a personal beef with the rest of the Titans, to Brother Blood, who started off perfectly menacing but spiraled into Villain Decay quickly and had limited interaction with any of the Titans besides Cyborg, and having a weak story that only got two episodes and a two-part finale worth of exposure when other arcs usually had one or two more.
To some, season 2 of X-Men: Evolution is this. Over focus on romantic side plots, less focus on their battles and less action. Thought the ending managed to fix that by revealing The Masquerade, leading to it Growing the Beard in season 3.
During the third and fourth seasons of KaBlam!, the production company for Sniz & Fondue went bankrupt and more one-shot shorts were produced, with mixed to negative receptions among the fandom, though the remaining regular shorts were claimed to be even better that season.
The series remained popular to the end, though there are many people who think that the show's quality went downhill during season 3. More accurately, the episodes from the middle of the third season just aren't good as episodes from the second season and the first half of the third. That said, the show remained quite good - it just seems to have peaked in quality sometime around the Big Damn Movie, Phineas and Ferb The Movie: Across the 2nd Dimension.
Some people cite the downfall of the show as far back as season 2, due to more emphasis on formula and redundant jokes as opposed to the show's first season which was a Slice of Life show mixed with comedy. In other words, it was Family Guy meets Hey Arnold! in its first season.
A few fans felt the show got stale around the time season 3 came around due to more emphasis on the formula and the writers had stopped developing the characters. Coincidentally around this time, many of the show's original writers left to work on Regular Show and Adventure Time.
Among Scooby-Doo fans, general consensus is that two series fit this. Firstly, The Richie Rich/Scooby-Doo Hour, which made the bold move of completely changing the show's premise and in doing so fell flat on its face. Fred, Daphne, and Velma were inexplicably gone, the series was switched to a Three Shorts format, and the mystery-solving plots and Scooby-Doo Hoax were jettisoned in favor of slapstick chase scenes featuring Scooby, Shaggy, and Scrappy running away from real monsters for five straight minutes (the real monsters, incidentally, don't seem to have been part of the problem, as laterinstallments featuring real ghouls were received much better). Second, Shaggy & Scooby-Doo Get a Clue!, which also tossed out the mystery premise and Fred, Daphne, and Velma, but not only that, got rid of the distinctive Hanna-Barbera art-style and horror elements entirely, in favor of, of all things, a spy thriller. The Scooby-Doo and Scrappy-Doo Show sometimes gets lumped in there too for introducing... well, guess.
While The Avengers: Earth's Mightiest Heroes! has several great episodes in its second season, it also suffers from having more filler than the first, especially after Jeph Loeb and Man of Action Studios came on as executive producer and creative consultants, respectively. They created so much filler, that subplots the original writers set up earlier went unresolved and/or unexplored by the time the show ended. Their run also saw most of the Avengers get pushed Out of Focus, and Out of Character Moments became more frequent. Plus, the animation in this season sometimes seems cheaper, and the awesome theme song permanently got ditched in favor of recaps of old episodes and a promo for the Avengersmovie.
Many King of the Hill fans consider season 9 the show's nadir, due in large part to the massive retcon of Peggy in the premiere "A Rover Runs Through It" and Lucky becoming a regular cast member ("Care-Takin' Care of Business"). These along with ongoing Flanderization (especially of Luanne) and a lack of well-regarded episodes make it a season few fans stick up for.
While hardly bad, season 3 is often considered the weak point of Metalocalypse, due to a rather severe Myth Arc stall and the series not being allowed to use its traditional Gorn due to the network fearing it would be perceived as too similar to Superjail. The lack of gorn was replaced with a noticeable increase in Vulgar Humor. Season 4 was much more in the spirit of the first two seasons. The episodes were also twice as long (half-hour instead of quarter-hour), which while a good idea in theory, resulted in very noticeable pacing issues. Along with that the Charles Ofdensen fans weren't too happy about his severe lack of appearance.
Fans of Dan Vs. consider season 2 to be this, due to too much focus on Elise, a lack of Black Comedy, and making Dan into a Failure Hero (when one of the best parts of the show was watching him succeed). Season 3 is considered an improvement, but not by much, which is why the show didn't get renewed after that.
Schoolhouse Rock: Earth marked the first time in seven years that the crew released new songs. Unfortunately, almost none of them, if any, hold up against the classic songs. When counting only the seasons that aired on ABC, Scooter Computer and Mr. Chips would get deemed the weakest. It didn't even get a home video release until Disney started releasing Schoolhouse Rock songs on DVD.
Robot Chicken has two fan-noted cases of this: Season 2, which is considered weaker than the sublime first season (though fan opinion of it has gotten better over time) and the infamous Season 5, which saw the show's usually witty humor go completely out the window in favor of over-the-top violence, crude Toilet Humor, and too many Dude, Not Funny! moments to count. Apparently either the writers were listening or replaced, because Season 6 took a sharp turn in the other direction, much to the delight of the fans.
There are a lot of fans of Thomas the Tank Engine that have diverse opinions on when the series went downhill:
Some say it was when the show became less true to the Rev W. Awdry's original stories. Awdry himself disowned the show following season 3, when writers took more liberties with his stories and also started created their own, which sometimes disobeyed railway code and had inaccuracies to the story mythos. After Season Four, adaptions of the original novels ended altogether.
A lot of fans were also disappointed when Britt Allcroft stopped producing the show and HiT Entertainment took its place and when the latter converted the show's format to CGI instead of using models.
A lot of fans pinpoint Sharon Miller's run as head writer (Seasons 9-16) as the low point of the show, due to stories being aimed towards much younger audiences, rampant rhyming and alliteration, the narrator always explaining the action as if the audience can't figure it out on their own, one-off characters being introduced for the sake of merchandising, the most evident amount of railway inaccuracies, very poor writing ("His smokebox was on fire" comes to mind immediately), and worst of all, it became clear that Hit Entertainment saw the series as nothing more than a marketing machine, and it effectively became a 30-minute toy commercial. Season 17, where the series was taken over by Andrew Brenner and reverted to more mature storylines, has gotten a warmer reception thus far, with several Miller-era characters being fleshed out (Paxton, in particular, has become a fan-favorite), the writing quality improving, the comedy being better-written, the animation using much more dynamic angles and clever transitions, the reintroduction of several fan-favorite characters such as Duck, Bill & Ben, and Harvey, better vocabulary and explanation of railway terminology, and overall, a better sense of respect for the viewers and fanbase.
The second season of Superjail is usually vocally met with scorn and invocations of this, if not just disappointment. Criticisms ranged from the animation being "too fluid", the characters suddenly having backstories revealed, some changes with the characters' personalities, and the story formulas changing. Most notably, the episodes no longer all had wild bloodbath sequences, and some had a pun or a spoof Aesop tacked on to the ending.
While Season 3 is looked at a little more fondly due to the crew attempting to merge the styles of both previous seasons, the change in animation studios (necessitated by Augenblick bowing out to work on Ugly Americans) and the Warden becoming too childish are still subjects of criticism.
While the Superfriends series was probably never "good", the nearly-universal consensus is that it gradually got better as it progressed, with the first season being particularly dire. (How dire? The introduction of the Wonder Twins and Gleek was a step up.)
Gargoyles fell into this after Greg Weisman left at the start of Season 3. Xanatos and Fox became full good guys and every villain was a Card-Carrying Villain with little to no depth. Not helping matters was that the season's Big Bad was basically a male, human version of Demona, but while Demona's reasons for descending into villainy were understandable, fleshed out and even sympathetic, this guy's reason for turning evil is entirely selfish on his end, runs on Insane Troll Logic, and doesn't get nearly as much detail as Demona's Start of Darkness did.
Batman The Brave And The Bold ended with its third season due to the show's creators wanting to avoid decay. That said, the third season is still considered to have taints of this trope, due to contention about the quality of the plots and Batman occasionally veering from The Ace to Canon Sue.
While not as much as the uncancelled SpongeBob episodes, when people say Rugrats went through this, it leads to a Broken Base. Most people say it's either after they added Dil or Kimi, some people say after it got Un-Cancelled, due to more emphasis on Toilet Humor and not as much Getting Crap Past the Radar and due to Arlene Klasky not letting Paul Germain (the showrunner of the first 3 seasons) know that the show was coming back and the show lacking his influence.
Most fans of All Grown Up! say that it declined after Season 2 due to more drama and less comedy, Flanderization, an Art Shift, and more unrealistic plots.
The series, while not as much as Family Guy has been said to have a small/minor decline in Season 7 for less comedy and more drama as well as some character's traits becoming more exaggerated (especially Roger). It's also disliked for having boring plots with glacier-slow pacing and having most of the actual comedy reduced to over-the-top gore and bizarre big lipped alligator moments that you'd expect on Family Guy.
Season 10 is accused of this, namely due to the notably reduced focus on Stan in favor of the rest of the Smiths while Stan is relegated to the center of the B-plot.
The Dreamstone's first season is often better regarded due to its darker plotlines and character designs which are more loyal to Mike Jupp's original concept (along with suffering less from Executive Meddling forcing more side characters). The later seasons however do get credit for aspects such as more fluid animation and remedying some of the earlier episodes' flaws (eg. giving the heroes more focus and downplaying their Unintentionally Unsympathetic facets against the Urpneys).
Many fans of Animaniacs consider the show to have generally weakened after the Channel Hop from Fox to Kids' WB, as not only did the writing seem to take a nosedive, so did the animation — the "spark" just didn't seem to be there to make many of the Kids' WB episodes as entertaining, while the animation went to weaker studios for the most part. They also lost Pinky and the Brain which didn't help one bit.
Regular Show: Season 4 is considered this by more than a few fans due to recycled and/or dull plots, the Romantic Plot Tumor of Mordecai and Margaret, and Muscle Man becoming a one-man Spotlight-Stealing Squad. Season 5 did resolve most issues from season 4 by restarting a new romance plot with CJ from season 3, removing all Muscle-man centered episodes to give him only one or two episodes like Tower Power, and using most new plots and giving some more character development for lots of main characters.
While "rot" might be a strong term, it's common agreement that season 5 part 2 (especially the first half) is not to par with the rest of the series quality-wise. Featuring the breaking up of Finn and Flame Princess (which is still spawning Fix Fics and shipping wars), the increased involvement of the increasingly creepy Lemongrabs in plots, Princess Bubblegum's moral ambiguity beginning to border on full-blown sociopathy and possible villainy, and Finn, of all people had Took a Level in Jerkass, with his immaturity being played up to ridiculous levels to the point he was Worfed in "The Red Throne" and lost any chance of getting back together with Flame Princess. Thankfully, the season did recover in the second half with the debut of Betty and the reveal that Finn's father is still alive.
Season 6 got a bit of this too, for the lack of focus on its plotlines and ending in a fairly big anti-climax. The two-part season premiere, though, was well-received, if only for advancing the plot, Ron Pearlman's sinister and chilling speech, and, after much foreshadowing, Finn losing his arm.
Season 3 of The Transformers isn't as well remembered among fans as the first two seasons and the movie. Taking place after the movie, it is a radical departure from the previous two seasons, with most of the action taking place in space, many old characters popular with fans disappearing with new ones taking their place, the already poor-quality animation largely getting worse (mainly due to most of the season's episodes being animated by AKOM, rather than Toei) and the Decepticons sometimes being downplayed as villains in favour of other villains such as the Quintessons. However, a few episodes of the season remain some of the best-written and best-animated of the series, such as "Call of the Primitives", "Webworld", "Dark Awakening", "Dweller in the Depths", and "Chaos".
Season 3 of Beast Wars is often seen as a drop in quality compared to S2. The mostly serious but occasionally healthily zany tone became very inconsistent, with overly dark moments leading straight to corny slapstick gags. A lot of episodes were devoted to introducing new characters or upgrading older ones, leading to pacing problems and no sense of an overarching plot. The season undid Inferno's on-screen death at the end of S2, without giving him a reason to continue existing (other than being Flanderized more). Optimus Primal was made too powerful and had to be written out of most episodes, making him come off as a wimp most of the time. A planned dark, story and character-centric episode was even replaced by a humorous yet pointless filler that lead to something of a plot-hole in the series finale. Despite these, and despite many fans thinking that Simon Furman's script for the final episode "missed the mark" on the show's tone, the series finale is still often regarded as one of the best in the franchise, and visually, this season had the best graphics.
Season 2 of Transformers Prime is generally considered to be the weakest out of the three. Due to the writers combing two seasons of storylines into one, the season came out very cluttered and rushed. Plotlines that were considered interesting (like M.E.C.H, or Bulkhead's near-fatal injuries) were not given enough episodes to properly develop before they were quickly resolved. Several fan-favorite characters were also killed off for the sole reason of proving that Anyone Can Die, even if there were still stories that could be mined from them (in particular Breakdown and Dreadwing). While there was enough stuff that was worth watching, overall the poor pacing really turned a lot of fans off. Season 3 was considered an improvement over season 2, mostly for the better pacing.
The third and fourth seasons of Dexters Laboratory were made after creator Genndy Tartakovsky left for Samurai Jack. While some of the writing was still pretty good, the art (particularly the character designs) were standardized into a simplified, blander style, the animation became less dynamic, and the production values in general felt rather cheap.
When Batman: The Animated Series was retooled as "The New Batman Adventures", the character designs and visual style were altered to fit in with the emerging DCAU (particularly Superman: The Animated Series). Much of the hand-drawn charm and film noir flair were lost. While the final season still had some classic episodes, the writing was also rather hit-and-miss.
The final season of Chowder is often cricticized by fans thanks to Flanderization of a number of characters (such as Chowder's lack of an indoor voice and smarts, Panini's stalkerness reaching disturbing levels, etc.), the Fourth Wall gags becoming overused, and in general just not being nearly as clever as the earlier episodes.
The seasons of Arthur from 16th onwards are disliked by many fans because of the use of Flash animation instead of the traditional drawn animation used since the beginning of the series. Petitions have been made to change the animation. Complaints were also made about the addition of a new character, Ladonna, to the main cast.
Although while still good, the fifth season of The Venture Bros. was considered disappointing by some fans. It was a short season so they didn't really have a chance to flesh out some of the plot points. A part of the reason is also because the show is praised for massive changes and character development, while this season undid some of that and made certain things go back where they started. The Monarch had a smaller role too.
Betty Boop went into a steep decline around 1934; first the Hays Office neutered the series raunchier elements (downplaying Betty's sexual qualities and prompting the abandon of her dog boyfriend, Bimbo), and then the Fleischers began to incorporate ill advised changes to the series to cuten it up, namely introducing rather dull characters such as the ever cloying Pudgy, Betty's pet dog (who would often headline episodes by himself along with being a co star to Betty), and Fearless Fred, her one note heroic boyfriend, and downplaying the series once wild animation and gags into much blander, down to earth premises. The series ran on fumes a few more years, and it was finally ended in 1939.
By the mid-1940s, the Famous StudiosPopeye shorts became increasingly formulaic and stale, and the timing and animation took a hit in quality. By the 50's, the series went through such a clear budget crunch that they were forced to make an excessive amount of clip show episodes or remakes of older shorts. Roughly 17% of all Popeye theatrical cartoons from both Fleischer and Famous Studios were either remakes, semi-remakes or clip shows, that's roughly 38 cartoons in all! However roughly only 3% (4 total) of the Fleischer cartoons qualify, whereas a whopping 28% (roughly 34) qualify for Famous Studios.
The post-movie seasons of G.I. Joe: A Real American Hero, which changed animation studios, writers, and replaced almost all of the voice actors from the Sunbow seasons with ones from The Ocean Group. The animation's budget was much lower than Sunbow, and the plots became even sillier than the early episodes.
Ctrl+Alt+Del: The Animated Series was a pretty miserable show to begin with, but the second season managed to miss even the low bar of the first. The animation lost all shading and what little fluidity it once held, errors became more and more common, and the episodes became considerably longer, leading to great heaping amounts of Padding.
Due to the show's Merchandise-Driven nature, the second season of M.A.S.K. switched formats from "heroic M.A.S.K. agents vs. evil VENOM terrorists" to 'M.A.S.K. and VENOM as rival racing teams." Many fans were turned off by the sheer illogic of the change.
While season 2 was already polarizing, The Amazing World of Gumball is believed to have declined with season 3. While it isn't without its good episodes, there seem to be a lot more mediocre/bad episodes that season than the past two and some major flanderizing among much of the main cast (especially Darwin and Gumball).
Sealab 2021, with the second half of its third season and fourth seasons. One major culprit of the show's decline was the removal of Captain Murphy (due to the death of his voice actor), who was seen by many fans to be the heart and soul of the show's Golden Age.
The second season of Jimmy Two-Shoes is often considered worse than the first one. The main culprits were going through Flanderization and a massive downgrade on it's animation quality. A few felt that the show lost a lot of the Season 1 charm through decent writing becoming mediocre and less memorable episodes too.
Johnny Test. The first season is generally seen as decent. After it finished its run, the show was acquired by new owners, which led to the entire production staff working on the show being replaced and consequently the show's animation changing from traditional to Flash. It is generally agreed that the second and third seasons, while taking a hit in animation quality, were still at least watchable, but when the show was revived for a fourth season, everything went to hell. The animation went from "mediocre but passable" to "not trying at all", the sound effects became cheaper (the infamous whipcrack noises were never used in the first season), and Flanderization destroyed whatever human decency was left in the cast. It's quite easy to see why the show is universally hated when one compares Season 1's quality with Season 6.