In the case of the Oh My Goddess! manga, it's "Growing The Ponytail". The series began somewhat crude and crass, and the main cast was comprised of only Keiichi and Belldandy, who then had long, silvery hair. As the series evolved and more characters were added, Kösuke Fujishima started making her (and her sister goddesses, Urd and Skuld, when they joined) look less Asian and more European. By the time Fujishima had gotten a firm handle on the character designs (with Belldandy in blonde hair in a bun and ponytail, with some dangling for good measure), the series had more or less become a light-hearted romantic-comedy.
Interestingly enough, the OVA (which debuted while the manga was still Growing the Beard) and the TV series (which debuted well after the manga had settled down) have Belldandy in a more European-style blue, gold, and white outfit the first time she appears, and with blonde hair in a bun and ponytail, and both series follow the light-hearted romantic-comedy formula to a "T". In other words, the "beard" had already been grown.
The AKIRA manga picks up a lot of steam at the point where Akira demolishes Neo-Tokyo, transforming from an edgy cyberpunk story about unravelling the mystery behind a group of psychic kids to an intricate, psychologically and politically riveting post-apocalpyse epic. The animé ends just at the point where the manga gets much richer and more involved. This may have been a wise filmmaking choice, since it would have taken what, four, five hours to even dent the plot of the last half of AKIRA?
The Axis Powers Hetalia was always a light hearted and cute Anime, but the third season (Named independently from the first three as "World Series") has a distinct Darker and Edgier feel, with darker colouring, more visible lines, better quality and more fluid animations, along with more original plots to go alongside the ones from the manga, it's also much funnier than the first two seasons.
Not to mention it now contains PRUSSIA! Which probably adds to the humour department.
The fifth season seems to be repeating this process. It's got a new name again ("The Beautiful World") and a new team with a noticeably more polished art style and has become a bit more focused (generally adapting only one story per episode, where episodes in earlier seasons would skip around between multiple storiess).
Berserk: around volume 9 where stuff starts hitting the fan.
The Manga's first arc (which looks like shallow torture porn at first) starts to grow the beard when the snail count visits his daughter. It's the first sign of the layered storytelling that the manga currently is known for.
Speaking of Berserk, the anime had a well-written but very slow start. It really gets interesting at episode 15.
Blood+ gets a bit better around the time Saya grows her hair out. It makes sense since so many things happened right before her hair style changed: Her younger foster brother is killed, Red Shield's HQ is destroyed, its leader crippled, and her angst filled Quest for Identity is finally completed. Not that there wasn't more angst, but the second half of the series was both more entertaining and more memorable.
Blue Gender. The first ten episodes or so, out of twenty-six, are set on Earth. They're relatively lackluster, feeling more like a rip-off of Starship Troopers with a touch of Mobile Suit Gundam than anything else. They're also very episodic, and you can easily skip most of the first half... then Marlene and Yuji get to Second Earth, and the series takes a sharp turn towards fucking awesome. The introduction of Yuji's Evil Counterpart, Manipulative Bastard Tony, helps. The introduction of B-cells helps too.
CLANNAD was always a decent high school romance comedy, but it REALLY proved its depth at the end of episode 18 when the other girls realised they had to give up on Tomoya because he loved Nagisa, and in episode 19 when Tomoya ran away from his broken home and moved in with Nagisa's family.
Similarly, after ~After Story~ trimmed its beard by falling back into high school filler for the first eight episodes, it came back with a vengeance in episode 10 when -get this- Tomoya graduates and gets a job! Not the kind of thing normally seen in high school romance comedy, is it? The beard grows even longer in episode 12 when Tomoya asks Nagisa to marry him and by the end of episode 16 when Nagisa dies in childbirth this series has the kind of beard you expect to see in The Guinness Book of Records.
Though the episodes before it had had their highlights, the conflict at Narita in episodes 10 & 11 of Code Geass marked the point at which the plot hit its stride and entertaining plotlines were produced both for Zero's rebellion and Lelouch's life at Ashford.
Digimon Adventure started out as a fun, if typical, Trapped in Another WorldMons series. Things picked up at around episode 21, but it wasn't until the introduction of Vamdemon/Myotismon (an incredibly iconic villain for children's anime, even after a decade) that the series really hit its stride.
Digimon Tamers: After some slow, character-building episodes, the show grew it's beard with episodes 13 and 14, as the Director finally reveals the breadth of his roots in Horror to give a whole new kind of Digimon anime that's heartwrenching, terrifying, and awesome.
Digimon Savers starts off as "GeoGreymon victim of the week" series but starts to pick up with the introduction of Falcomon. Then Kurata shows his hand and the fun and genocide begins...
Digimon Xros Wars showed great improvement by the introduction of DarkKnightmon and began to really hit its' stride with the commencement of the Death Generals arc, the reintroduction of traditional evolution and the conflict with Yuu Amano.
Eureka Seven was a quirky but tepid mecha anime for most of its first half, and most of its watchers were rather ambivalent towards it on its first [adult swim] airing. Then came the first-season finale, which, along with a marked increase in animation quality, set the stage for the rest of the show and was a wonderful Crowning Moment of Heartwarming in its own right. Then the real fun began...
According to the Word of God and That Other Wiki, this was planned by the writers in advance: first give us a shonen-y love drama and then without a warning, turn the whole show into an epic of mind-blowing proportions.
Fairy Tail got way more serious and darker in the Tartarus Arc. Just to make a point, up until Tartarus Arc, only a few characters got killed during the series, and Hiro Mashima was widely recognized as being an author that did not kill his characters. Five chapters into the Tartarus Arc, and one of the members of Tartarus made an attack at the Council, killing all members but one, and a few chapters later, defeats even Natsu. Another one used her powers to kill possibly all the members of all dark guilds associated with Tartarus. Not much later, another member kills more than 100 people, and incapacitates the entire Raijinshuu Tribe. Another member was able to freeze and defeat a dragon, something not even the Dragon Slayers themselves were able to do. The guild master is also the strongest demon ever made by Zeref, which not even the king of the aftermentioned dragon was able to defeat. It's safe to assume that the overwhelmingly high praises for the arc are not misplaced.
Fist of the North Star starts off as a series of loosely connected story arcs where one thing leads to another. It isn't until the introduction of Kenshiro's three honorary brothers in the ways of Hokuto Shinken and their dispute over who will become the successor is where the main story truly begins.
The manga itself did this quite early. The manga's first book seems to set the series as a Mystery of the Week format, where the two brothers journey around and solve various problems via alchemy. As soon as Edward and Alphonse reach Central and meet up with Mustang, the story turns into a longer single arc.
The 2003 anime also started off as a very high-quality though fairly typical Mystery of the Week series where Adventure Towns were the main source of the plot. However, around the time of Hughes' death, it finally started to evolve into a truly dark and compelling drama. By the time Ed fought and killed Greed, its beard had grown out to roughly ZZ Top proportions.
Full Moon o Sagashite does this in the anime. It takes a good 30 episodes of happy fluffy fun times (if you forget the protagonist is going to eventually become mute due to a throat tumour or, alternatively, die) before things start getting serious and very emotionally hard-hitting. The second half of the anime is very nearly unrecognisable from the first.
The first season of Fushigi Yuugi leads one to think it will be a Happily Ever AfterMagical Girl story. It's slightly violent, a few deaths of Mooks and the like, but the main characters always survive unscathed... until the first season finale, where the male lead's entire family is brutally murdered. From that point on, the series takes a much darker turn, and most of the main characters die.
Gantz, for the first nine volumes (and the whole anime series), consists of senseless killings; gory, horrific violence; Fanservice; and sex scenes. Thanks to the introduction of Tae Kojima and impressive Character Development for Kurono, however, the manga has since improved immeasurably.
The original Getter Robo manga, despite being an influential and important series for the Humongous Mecha genre, is still a fairly typical shonen action comic with some shaky art. When the series was revived 15 years later in Getter Robo Go, now under the full creative control of Ken Ishikawa, things really started to take off.
Ghost in the Shell Stand Alone Complex has a few rather unremarkable episodes at the beginning, basically introducing the members of Section 9 and the technology they use. With the fourth episode, the Laughing Man story arc kicks in.
Mobile Suit Gundam ZZ has one of these around episode 20. Up until then, the show had been incredibly light-hearted and silly, to the point where enemy elite mooks were painting roses in the sky with their mecha before going out to get shot to death. But during the short arc with Cecilia at Granada, Gottn tries to blow up a shuttle full of poor people in order to trap the Argama. This backfires when Cecilia discovers she has the bomb needed for this, and sacrifices herself to blow up Gottn's ship. Mainly considered to be Cerebus Syndrome, though.
Mobile Suit Gundam SEED is rather slow-paced for the first 30 or so episodes (basically, every episode generally follows the ZAFT-attacks-Kira-saves-the-day routine; that these episodes are more or less a Remake of the original Gundam series doesn't exactly help), but after a few Wham EpisodesSEED finally escapes this routine and sets off on a path to its own original, epic Grand Finale. To its defense, the first episodes do a good job of familiarizing and endearing the characters to the audience.
The Anime adaptation of Gun Grave opens with the universally acknowledged shithouse episode 1. However the episode is important as it is in medias res and gives an idea of where Brandon, Harry, Harry's friends/followers, Big Daddy, Maria and the Necrolyze research ended up but not HOW. Episode 2 begins a flashback that takes up more than HALF of the series consiting of seriously epic drama.
GUN×SWORD starts a bit slow and episodic... and then at the end of the sixth episode, Van asks a waiter if he's seen a man with a claw - a question to which the answer has always been "no." The response is as follows: "Yes. He's right over there." That moment starts the show's momentum going, and it never loses it until the end.
For the anime, it could probably be said that this happens in either Tsumihoroboshi-hen, or Kai. During the former, the series finally stops making its protagonists turn into insane,paranoidvillains who murder the remaining characters before either killing themselves due to the hate plague or being killed by some outside force. For once it stops focusing on paranoia and murder mysteries and focuses more on the theme of friendship, which leads to the first happy ending in the entire series, not that it lasts long. The latter moment of growing the beard answers several unanswered questions of the previous arcs, while finally giving the show more uplifting moments, the characters no longer at each others' throats. While the arcs still have the characters dying in the end for the most part, they usually get in some awesome or heartwarming moments which, unlike in the previous season, seem to be more heartfelt than just plain silly. The true villain and the reason behind the time resets are revealed to the audience, and the group seems more passionate about their friendship, giving the viewer more hope.
While Inuyasha does get a lot of flack for its Arc Fatigue, many fans believe that the rebirth of Kikyo and the introduction of Naraku is when the series began to shift away from a simple quest for the Macguffin with Monster of the Week elements and focus on a more complex story involving the band of heroes facing off against a powerful and seemingly impossible to kill enemy, while experiencing developments as a characters.
JoJo's Bizarre Adventure begins as what could best be described as Fist of the North Star with vampires for its first two arcs (Phantom Blood and Battle Tendency). With the Stardust Crusaders arc it really comes into its own with the introduction of battlespirits called "Stands" and does away with the Hokuto Shinken-esque martial arts used in the previous storylines. And in Part 4, the art noticeably begins to shift away from looking like Fist of the North Star and develops a completely unique style. Not to mention that, in Part 3, Joseph literally grows a beard.
Jubei-chan had about 7/10 comedy and 3/10 action. Then the sequel came, improved animation quality and turned that ratio on its head.
Okay, Kannazuki no Miko is a Twelve Episode Anime, but it still grows a definite beard halfway through. Be patient with the silly mechas and shonen anime clichés — when Chikane's issues take centre stage, that's when things are about to get way the hell more interesting.
Katekyo Hitman Reborn! had a basically stagnant plot until Volume 9 with the arrival of Mukuro. The story and plot got a lot more interesting, hitting its stride during the Varia Arc and kept getting better from there (except the 'Future' arc which had many drop the series). Even the art got better.
The later chapters of Kodomo No Jikan tone back the overaggressive lolicon antics in favor of much darker storylines deconstructing the reasons behind Rin's behavior, as well as changing the relationship between Rin and Aoki such that the latter is actually concerned for the former rather than annoyed by the unwanted attention.
Last Exile spends the first three episodes with next to no plot development, only going into high gear around episode 4. Some fans push this ahead further to episode 7, when Dio is introduced.
The anime version of Lucky Star suffered from a very poor director in its first four episodes. It was bad enough that the producers actually canned him and had him replaced, after which the series takes a noticeable turn for the better, with the sixth episode being a Beach Episode. Unfortunately, this is like many other cases in that it's hard to get your friends into it when the first episode spends twenty minutes talking about how to eat a pastry.
Macross 7 picks up around episode 17, with the separation of City 7 from the fleet, and picks up once again in episode 27 with the creation of Sound Force.
The trend is continued in the following seasons, A's and StrikerS, as the story transforms into a psychologically complex, multi-dimensional saga.
Magical Project S: Although it parodies the magical girl genre, never takes itself seriously and taking into account that this is a subjective trope from episode 19 you can see that besides the ever present jokes, there is an actual change of the status quo thanks to character development. It goes beyond being a parody show and adds psychological complexity to some characters (particularly Misao/Misa) that lead and from this point the characters were taken seriously beyond (but retaining) comic relief.
The Mahou Sensei Negima! manga does this around volume three, when it begins to switch from an Unwanted Harem comedy to an action series with the arrival of Evangeline. It really hits its stride at the end of the Kyoto Arc, around volume 6. Unfortunately, the anime adaptation didn't get that far.
Fans of Mai-HiME believe that it grew its beard at its eighth episode, with events (the killing of Harry by Miyu and subsequent "death" of Kazuya) that kick off an Ancient Conspiracy's intervention and cause a breakaway from the hitherto-formulaic plot.
Monster really takes its time setting up its characters and situations, so that for its first 20-some episodes it appears to be a warmed-over Fugitive knockoff with a doctor on the run from the law helping people he meets while trying to find the real perpetrator. Then we get a complete shift in focus to Dr. Reichwein and some other new characters being affected by Johan's plans, with the series taking on a much stronger focus on uncovering Johan's backstory rather than simply chronicling Tenma's travels.
Muhyo and Roji gets quite a bit more interesting than a Ghost Of The Week series when Enchu is revealed as a Big Bad, but the series truly gets interesting after Rio is revealed as a traitor, when the plot shifts to the war against the evil Ark organization.
While the first arc of Naruto was well-received, the series started growing with the Chunin Exam Arc. We're introduced to the Big Bad of part 1 as well as many Ensemble Darkhorses, and characters such as Naruto and Sakura start showing their signs of Character Development.
Regarding the Shippuden anime, after the tragedy of bad animation throughout Pain's Invasion arc, and after the subsequent Filler Hell before and after the Kage Summit arc, the series started picking the hell up. First was Naruto vs. Kyuubi, an incredible climax to a lifelong battle. Then came Naruto's birth story, which had movie-quality animation and a new OST to boot. After that came the Fourth Great Ninja War, complete with new CG rendering in scenes (due to the tens of thousands of people/Zetsus).
Speaking of Fillers, while once very clumsy in execution, have gradually gained more and more Hidden Depths, Character Development and flowing story starting with the 4th Fourth Ninja War, especially with the new "Power" arc which has great movie quality animation and the use of subtle writing, along with Character Development and fighting sequences, with each episode bringing a new surprisingly shocking, but foreshadowed developments that turn the story in a better direction with each episode; especially if you compare it to the Filler of Part 1 and almost all the filler episodes prior the 4th Great Shinobi War.
Starting about halfway through the series (roughly around the 14th episode), the legendary anime Neon Genesis Evangelion abruptly swaps over from a lightnote -ish-hearted scifi shonen series about super robots fighting towering, nightmarish creatures, with special focus on its characters' relationships and insecurities, to a noticeably more postmodern, existential, and psychoanalytical work of character drama and art-text-mystery play. The robot-vs-creature fights, fewer and further between, become substantially darker and more traumatic, increasing emphasis is placed upon disquieting mysteries seemingly surrounding everything, and beloved characters who had been shown to be emotionally fragile and dependant are gradually revealed to be downright psychologically broken, or are emotionally tortured until they break. Eventually, around the last two episodes, the series grows its beard out to truly impressive lengths and abandons all its own narrative precedents to become a kaleidoscopic investigation into the psychology of the main character, thereby codifying the Gainax Ending. It ultimately becomes an experimental deconstruction of shonen anime and escapist fantasy fiction, exploring the symbolic relationships between the Humongous Mecha, the characters, creatures they fight, the organization that created the mecha, and the audience and creators of the show itself. It was these changes in tone that would provide Evangelion with much of its most rabid and enduringly massive fanbase, and bring it unprecedented levels of cultural respect both generally and, especially, within the humanities crowd, which at the time was extremely rare for a shonen anime.
Noir. The first few episodes were basically filler. The sixth episode "Lost Kitten" was a truly touching moral dilemma. After that it was crime thrillers and ancient conspiracy and female gunslinging action to the climax.
Nurarihyon No Mago started off as a fairly average shounen manga, about a quarter-youkai boy who's meant to be the next head of a youkai clan but just wants to live a normal life. But once he stepped up to accept his role as their leader, much of its fandom agrees that the story took a sharp turn upwards.
One Piece was always fairly unique as a shonen manga with its blend of action and humor. The first few arcs are decent, but not terribly notable. This changed with the Arlong Arc, combining great action scenes, suspense and character development that captivated the audience and made them really want to see Arlong rightfully ground into the dirt. And it's only gone up from there. This is the source of a common advice for people considering if they want to read it or not is "Read it up to the Arlong Arc. If you don't like it after that, it's not the series for you."
For that matter, throughout the series the Big Bad Blackbeard himself has been growing a beard.
The Enies Lobby arc is when One Piece had really hit its stride. At that point, One Piece was considered a fairly decent manga/anime with good storylines. Then came along Enies Lobby, which had epic fights, great characterization, and a truly amazing moment, where the crew declares war on the world all for the sake of one of their own. If you don't like it after that, then it's not for you.
The One PieceTime Skip, starting with the Return to Sabaody arc, is seen as a restart of the Straw Hat's adventuring; being subject to yet another Growing the Beard moment as they start their journey through the New World. Many fans agree that the Time Skip era started to gain epicness during the Punk Hazard arc due to it's unpredictability and because the primary side-characters are finally getting involved. It helps that it's occurring just after the Fishman Island arc which many fans agree is perhaps one of the weakest arcs in all of One Piece. Fishman Island not only consisted of a fodder group (Hody's Pirates) that posed no threat to the Straw Hats what-so-ever, but was subject to excruciating Padding in which nothing got accomplished. Just how long were we going to have to watch the Noah fall towards Fishman Island or sit by as the Straw Hats beat the living pulp out of random fishmen Mooks?
Ouran High School Host Club does this in the manga. It starts off as an Affectionate Parody and slowly begins to grow the beard as we learn more about the characters and their past. As the story has gone on we've seen considerable character development, particularly with Hikaru and Kaoru. Tamaki also matures a bit (while still remaining the lovable idiot), and Honey and Mori finally graduate as the series falls out of Comic Book Time. Despite the story turning more toward drama, it hasn't sacrificed the humor.
The first few episodes of Panty & Stocking with Garterbelt were by no means bad, but the show found its true balance between action, comedy, drama, and pop-culture references when Scanty and Kneesocks arrive in episode 6.
Prétear starts out as a rather lighthearted, episodic Magical Girl Show, at least until Takako shows up. Afterwards it takes a huge turn and shifts into darker territory before the lightheartedness returns at the end.
Princess Tutu is a good, but fairly typical Magical Girl series — until Kraehe shows up. And if THAT doesn't convince you, wait until after the Happily Ever After ending of episode 13. It doesn't stay that way for long.
Puella Magi Madoka Magica starts off as a thoughtful, if somewhat ominous Magical Girl series. Then, come episode 3, viewers were left screaming "Did Mami's head just get bitten off by that cakeworm!?", at which point it became clear just what sort of series they were dealing with. Word spread out of the mahou shoujo/general anime viewership with intense speed and it is now considered one of the best anime of recent years.
Matt Kamen wrote, "With its...daring approach to a dated genre, Puella Magi Madoka Magica essentially does for magical girls what Neon Genesis Evangelion did for giant robots." (From the other wiki)
Rosario + Vampire started as a lighthearted, Monster of the Week, Unwanted Harem comedy with relatively little depth to the characters. Starting somewhere around the Witch Hill arc and introduction of Ruby, the series had already started to develop a more realistic artistic style and more dangerous villains. With Witch Hill, the series took a huge swerve by entirely chucking the whole Monster of the Week thing in favor of significant character development, the ongoing backstory of Tsukune's ghoul transformation, multi-issue story arcs, blurring the line on how "evil" many of the bad guys are and focusing on some truly depressing subjects. Season 2 is flat-out seinen with little resemblance to the series start.
Rurouni Kenshin's first season, while mostly good, is more episodic, occasionally silly, and bogged down with filler. The drama and character development ramp way up with the onset of the Kyoto Arc. Which starts with episode 28, so it's actually quite a lot of silliness and villain of the week that you're being asked to sit through before you even get a whiff of the real plot. The drama, character development and plot begin during the first Aoshi/Kanryu arc. It does take a break later before coming back in full in the Kyoto arc though.
R.O.D. The TV starts off very slow and episodic, with only the most superficial of connections to Read or Die and a strange amount of time spent on Anita going to school. Then the show gets a kick in the rear when the entire Ancient Conspiracy of the British Library comes into play beginning with kidnapping Nenene and the destruction of the entire island of Hong Kong, and the main plotline becomes the focus and not the subplot.
Sailor Moon spends its first volume on episodic mystery stories whilst gathering its main characters, mainly in the flavor of a superhero/detective story and then begins to grow its beard with Sailor Venus' entry into the story, Sailor Moon's revealed status as the Moon Princess, and Mamoru's subsequent capture and Face-Heel Turn. Aside from these three major characters bringing full focus to the story's plot, the manga also does a Genre Shift on the whole taking itself completely away from episodic superhero/detective stories and becomes a full-fledged mystery/fantasy story for the rest of its run. Rather humorously, Usagi's hair grows rather long as a result of her awakened identity parallel to the story's grown beard.
The main series gets this treatment when the Silver Saints make their appearance.
Saint Seiya Omega Grew a beard in episode 8, when the bearded Iolias, Golden Saint of Capricorn revealed himself as a Villain and a traitor, he is pretty damn badass for his age beating the snot out of Pegassus Koga.
SHUFFLE!! really got better and more dramatic with the beginning of Nerine and Lycoris' arc.
Tenchi in Tokyo's low point has to be the thirteenth episode, 'Moon Mission'. Starting on the next episode however, the series takes a turn for the better, going exploring the family dynamic as the girls go their separate ways due to Yugi's plans.
Transformers Armada started out as a sub-par Gotta Catch 'Em All series, not improving at all until midway through the show's run when they did a heel-face turn with Starscream (who became a much more shades of grey character as far as honor went) as well as introducing Armada's surrogate Starscream, the villainous, backstabbing Thrust.
The improvement first began with "Rebellion", when Sidways really starts to manipulate. By Starscream's Heel-Face Turn, the beard is full and bushy. By his SECOND Heel-Face Turn, it's like the show's face is one bristly hedgehog full of beard hair. Then Unicron showed up, also giving us the first Unicron toy ever to reach mass release.
Also applies to Transformers Cybertron. After Armada had finished its run and been replaced by the dismal series known as Transformers Energon, fans were pleased to see humans who weren't annoying, a plot that made sense while staying strong throughout its run, a decent dub and enough CMOA's to make a very decently sized page.
Trigun's first few episodes can be accurately described as "The Wacky Adventures Of Vash & Friends". That all changed in the twelfth episode, which set up an overarching plot to a previously fillerish show. note This marks the point where the manga moved from a shonen to a seinen magazine.
Tsubasa Reservoir Chronicle changed from a lighthearted series full of ShoutOuts to past CLAMP series into a much darker story once the group arrived in Acid Tokyo and several startling character revelations took place. Main character Fay also physically exemplified this trope by allowing his hair to grow out after this story arc. IF you consider Darker and Edgier to be better. Some fans were put off by the unrelenting gloominess of the next three arcs, but the complexity of the plot and characterization certainly mushroomed.
Umi Monogatari starts rather formulaic, with a fair amount of fanservice (especially surrounding Marin) and a monster of the week pattern—until the mood whiplash halfway through, when the story becomes much darker and laden with symbolism about intimacy and separation. The excellent music helps too.
There are several things about early episodes of Urusei Yatsura that make them less well liked by fans of the series than subsequent ones. The art style is very primitive, the scripts contain primarily simplistic slapstick humor, and the majority of the episodes consisted of two separate stories (each occupying half of the episode's running time). After about 20 episodes, the drawing style began to improve noticeably, the two-part episode structure was dropped, the humor became more subtle and sophisticated, and there were occasionally more serious stories.
Witch Hunter Robin was rather blah through its first twelve episodes or so, featuring a rather episodic plot involving the protagonists hunting down random witches in an almost Monster of the Week format. All that changed during "Loaded Guns", when Robin's organization betrayed her, using her roommate Toko as bait for a trap that nearly led the titular character to her death, an action which kicked off the main plotline of the series. Since Witch Hunter Robin was a single-season show it's probable that this was deliberate.
The Yu-Gi-Oh! manga originally is a fairly episodic series where Yami Yugi plays various dangerous games against one-shot bad guys to punish them for their evil natures. However, when former one-shot bad guy Seto Kaiba returned as aBig Bad and started an Amusement Park of Doom, it resulted in a story arc in which Yugi's friends finally learn of Yami Yugi's existence. From then on, things became much more story-oriented and dramatic.
The anime focusing on Duel Monsters, starts off as a Monster of the Week kind of duel, which lasts till the Battle City. At this point, the story becomes much more arc-reaching and more complex and the animation drastically improves.
YuYu Hakusho goes through this twice: the first notable increase in quality is when Yusuke comes back to life and becomes a Spirit Detective resulting in the series becoming more action-oriented, and the second is later on when the Toguro Bros. and Sakyo first appear, thus giving the series its first major over-arching villains.
Medaka Box starts off as a Slice of Life story of the absolutely flawless student president, Medaka Kurokami going on quirky tasks of helping students with their issues, but once Unzen the head of the Student Disciplinary Corps shows up, the story and art drastically improved with its Genre Shift to Shōnen.
Majin Tantei Nougami Neuro was a Serial Killerof the week mystery solving case till Sai shows up and the mysteries become much more emotionally investing. It takes another leap with the HAL arc brings forth far more dramatic tension that what it had before.