Arc Fatigue: Arcs in Frontier tend to drag. The Velgemon and Cherubimon ones are long, but by the time the Royal Knights come around and curbstomp the leads episode after episode without much advance, some fans just put their foot down or skip to the ending. The writers were clearly trying to reduce the Plot Armor trope to the thinnest and create the most drama with what material they had, but it is common to feel they sometimes went just too far and made the entire series a borderline Failure Hero show as a result. Out of the first five recurrent enemies, Grumblemon was always a pain to merely last against even in random scuffles, Mercuremon literally one-shotted the Big Good of the series in the latter's very introduction, and Duskmon was just plain Nightmare Fuel; and for the last arc, after the heroes had received several power boosts, the Royal Knights wiped the floor with them episode after episode without any apparent advance, playing You Can't Thwart Stage One at its finest before the arrival of Lucemon. But even then, when the heroes had unlocked power to match Lucemon's, they continued taking scary beatings every time Lucemon unveiled a new form, and it ended up with the Legendary Warriors having to pull basically a Taking You with Me against him.
A sizable contingent of fans considers Frontier to be the franchise's Dork Age thanks to a combination of a Genre Shift into an animated Henshin Hero series, the abandonment of the beloved partner Digimon and thus the entire Mons concept, and the fact that it came right afterthe now much-laudedDigimon Tamers. On the other hand, a smaller contingent of fans praises it for taking Digimon back to its Adventure roots while still being its own beast, and at the same time for daring to go in new directions instead of recycling storylines and plot points from Adventure, as Tamers is sometimes accused of doing. The humans getting their hands dirty actually fighting instead of being a walking evolution battery for the enemy to exploit is considered one of the biggest draws of Frontier.
Related to the previous, there's some diversity on opinions about the show's storytelling. While one side praised it for doing an excellent job at making the villains a credible threat, especially compared to previous seasons, the other side criticizes it because this had the side effect of spawning potentially endless Filler arcs where the heroes would fail at achieving anything decisive, including an instance of one of the heroes being infamously defeated in her own power-up episode.
Cargo Ship: A portion of the fandom jokingly ships Koichi with stairs.
Complete Monster: Lucemon is a Fallen Angel Digimon with a messiah complex. In his own warped, self righteous mind, Lucemon thinks that the Digital World has gone rotten and wants to tear it apart bit by bit, bringing about the loss of much data and the deaths of countless Digimon, all so he can rebuild it in his image. He describes as a utopia where every Digimon that lives is stripped of their free will and free thought with Lucemon as their shepherd. In reality, what he intends to create is a Hell in Digi-world, where all Digimon are slaves and the only one who will be happy is him. To do this, he corrupted Cherubimon and made him the villain for most of the series. He then used his two Royal Knight followers to erase data from the Digital World, and once he was free, he disposed of said followers just to power himself up. He gets even worse when the Digi-egg containing his darker side turns into Satan Mode, a being filled with nothing but darkness and malice. In this form, he attempts to claim the human world as well. He also uses various methods to torture the good guys purely for fun. Lucemon was, in the end, completely insane and never thoughtful of the lives of others: only himself, his pride, and his hatred towards the Digital World's existence that was not up to his standards.
Continuity Lockout: Anyone who watches this series first will most likely balk at names like MagnaGarurumon and EmperorGreymon.
Creepy Awesome: IceDevimon in the dub. He's a freakish serial killer, with a Christopher Walken impression for a voice. But he's also badass and fun to watch. Duskmon is also pretty creepy, at least in looks, but he stole the show whenever he got in a fight.
Bokomon and Neemon are often cited as one of the highlights of the series, notably Neemon for his funny lines and antics.
While also being Fanfic Fuel, the unused "Fusion Evolutions" of Thunder, Ice, Wind and Darkness (Namely Rhinokabuterimon, Daipenmon, Jetsylphimon and Reichmon) are very well-liked by the fandom, notably because they would have help Izumi, Tomoki, Junpei and Koichi not being sidelined.
Fanfic Fuel: The unused fusion evolutions for the four other kids as well as the non-existence of fusion evolutions for the other elements (Wood, Steel, Water, Earth) which allow every fanart and fanfic possibilities. Not to mention the four lost kids the group find in a certain episode (Tommy's bullies) who are often used as teammates with the group with the aforementionned spirits.
Fanon: It's a commonly held belief amongst fans that the other group of children the group meets were intended to have the other four Legendary Warrior spirits. While a few hints of this can be found, it's never outwardly stated that this is the case.
Sakkakumon, known as Sephirotmon in Japan, is shaped like the ten aspects of God in Kabbalah, the Sephirot. Fittingly, that's the same number as legendary warrior spirits; and each orb contains one enemy corresponding with one of the ten elements. One of these orbs contains Mercuremon. Yes. Mercuremon exists inside himself.
Lucemon is full of symbolism. Not only is he clearly inspired by Lucifernote His name derives from the French word for "light", his attack is called "Paradise Lost Punch". Also, he has what looks like a Dharmachakra (Buddhist Wheel Symbol) on his chest in Chaos Mode.
The Elemental Powers of the first five Chosen Children? Same as those of the Five-Man Band from Smile Pretty Cure!, only the elements of the leader and the lancer are swapped. As if that wasn't enough, one of Smile's villains is a Wolf Man with a color palette mostly in cool colors (just like Wolfmon/Lobomon and Beowulfmon) and often gets paired with the light-elemental heroine in fanart.
The initial digivolutions are done by the team running their digivices along their hand... which Power Rangers Operation Overdrive did in 2007. (OK, Overdrive ran it along their arm, but close enough.)
Tai's voice actor, Joshua Seth, voiced a Wizardmon in one episode. When Seth retired from voice acting in 2005, some of his pursuits included being a comedian, hypnotist, and... magician. Really, he did.
So the whole story starts when a group of kids gets a bunch of weird messages on their phones that leads them to the Shibuya train station... heeeeey, wait a minute! Luckily only one of them is dead this time.
A good guy forcibly transformed into a bad guy with the color black as a color motif who makes a HeelFace Turn in a cyber sci-fi setting. Are we taking about Koichi or Rinzler/Tron? Bonus points for Disney producing both TRON: Legacy and the English dub of Frontier.
An odd duck in a franchise known for summoning monsters with computers switches over to transforming into monsters? Data corruption destroying the world? The voice of an Angel, voiced by Mary Elizabeth Mc Glynn, coming through the speaker, telling them what to do? A conflict that spills from a digital world to the real world? A fire elemental hero who uses the power of Agni and Vritra? The main character using the power of Ardha at the end? Two individuals identical except for hair and clothing as protagonists, only one of whom knows of the other at first, and is dead when he appears? Two years later, we get Digital Devil Saga.
The Pokémon Blaziken and Lucario resemble Agunimon and Lobomon respectively. Amusingly, this fits how several Pokemon from Gen 3 onward are commonly criticized for "looking like Digimon," inverting the original claims that Digimon ripped off Pokemon. Also, "human vs beast" is a common debate in the Pokemon fandom due to the controversy over the more anthropomorphic Pokemon designs.
Digimon Tamers introduces DigiGnomes as a separate digital lifeform from Digimon themselves. This series gives us Grumblemon, a literalDigiGnome.
Ho Yay: Several moments, like its predecessors do, but Episode 41 takes the crown. After Kouji saved Kouichi from LordKnightmon, they started talking very romantically. Complete with the Love Bubbles. Granted, that's just LordKnightmon's imagination, but that scene probably is the most slash-shippy scene in the anime. And yeah, that scene was one of the reason why the Twincest pair is one of the most popular ship in the fandom. Come on, even LordKnightmon ships Koukoucest.
Hollywood Homely: Calmaramon. From the characters' viewpoint, she's much less attractive than Ranamon and causes her Toucanmon entourage to ditch her. In reality, barring her squidlike bottom half, she's really not that hideous. In fact, she looks like a much more slimmed down version of Ursula. Her EX card◊ in Digimon Collectors even emphasizes that she can be just as cute as Ranamon is.
Ostensibly the leader of the evil Legendary Warriors, Mercurymon is the scheming Warrior of Steel prepared to undermine his own leader Cherubimon and allies for personal gain. Straying to the background to let his comrades do all the grunt work, Mercurymon seizes the chance to defeat Seraphimon in a single, bold Dark Reflection and steal his data for himself. Later, he uses his Beast form Sakkakumon to pit both heroes and villains against one another allowing him to copy their powers even when ShadowSeraphimon is defeated. Enough of a suave snarker to play Ranamon's jealousy to his advantage, Mercurymon was ultimately only undone when the DigiDestined unexpectedly overcame his own Thanatos Gambit. Ever prepared to construct grandiose spectacle and speak in that slick Shakespearean accent, Mercurymon was one of the most competent villains in the anime.
Murmukusmon is main antagonist of the movie. Arriving to the Island of Lost Digimon long ago, Murmukusmon obscured an ancient tablet about Ornismon to trick the populace into revering the monster as a benevolent legend. Posing as d'Arcmon and Hippogriffomon, leaders of the Human and Beast Digimon respectively, he strokes constant civil war through powerful rhetoric, convincingly feigned emotion, and appeal to their pain, causing many allies from all sides to revert into Digi-Eggs, and thus excess Digi Code, when defeated. By the time the DigiDestined discover his ruse, Murmukusmon acquires all the code he needed, reveals his deception and the true history, and initiates Ornsimon's revival. Cold and calculating, Murmukusmon played everyone for fools and schemed to conquer the Digital World with his new living superweapon.
Mis-blamed: This series is thought by many people to have been the cause of the Digimon franchise's decline in popularity and the long hiatus until Digimon Savers. In actuality, it was the end of the virtual pets fad (and thus the base for the Digimon franchise itself), poor merchandising attempts both in Japan and abroad, and in the Anglosphere's case, Disney's apathetic treatment.
Moral Event Horizon: Lucemon crosses it prior to the series by corrupting Cherubimon, driving him to gather all the data in the Digital World so he could absorb it and escape his prison, leading to all the destruction he caused during the series.
Seasonal Rot: Frontier is generally perceived as underwhelming when compared to every previous Digimon series, due to, among other reasons, the absence of partner Digimon for the children to bond with (opening less room for Character Development), clumsy storytelling with tons of filler and shock value, and a rather weak second half which left every other character besides Takuya and Kouji Out of Focus.
So Okay, It's Average: Probably the most consistent complaint with the series aside from the Henshin Hero thing is that it simply doesn't have much else going on, with many characters and story arcs feeling bland, recycled, or underutilized, and some arcs and battles carrying too much shock value. At the same time, however, few would call it as grievously flawed as the last part of Xros Wars or the low points of 02.
Stock Footage Failure: In the last episode, when the Chosen Children fuse together into Susanooomon, the evolution footage shows Takuya and Koji's first Digivices rather than their later upgraded ones.
Removing the Digimon partners and making the Digidestined more like Power Rangers was the biggest issue many people had with Frontier, which really adds more fuel to the fire of the Broken Base about the quality of the series.
A more minor one being the lack of the Digi-Rap as the English opening theme, which was due to the English dub rights changing hands - though admittedly, the new theme is kickass.
A Gallantmon appears as cameo in Autumn Leaf Fairs. That's right, a cameo, without any lines, in a series which introduced the freaking Royal Knights! The American dub fixes this a bit in that the Gallantmon speaks in the same manner as in Tamers and even refers to itself in the plural as a definite Shout-Out to Tamers, but that's still not much.
All of the other children had fusion forms, but they never receive them during the show, echoing how the original Digidestined from Digimon Adventure except the two leads never reached their Mega levels (at least until Digimon Adventure tri.).
All of the other children in general upon Takuya and Kouji reaching their highest forms. After this point, the other children end up just giving up their Spirits so the two could reach those forms and then standing around for most of the rest of the show. In the previous series, the protagonists that didn't reach Mega were able to at least still fight and thus try to contribute.
The spirits themselves could qualify. They were not shown to be Empty Shells for the children to simply transform into but rather unconventional partner Digimon that have to stay in the digivice due to their fragile state as ghosts. Instead of playing the role of Spirit Advisor (no pun intended) for their human partners, they were reduced to silent Plot Coupons to likely avoid copying Tamers. This resulted in one of the main criticisms of the show being a Human-Focused Adaptation, even more so than usual.
Takuya and Kouji outshoneeverybody. Yes, the other seasons usually had The Hero and The Lancer gain the highest evolutionary forms, but this series was especially egregious with this. What makes it worse is that the beast and human fusion hybrids for the other Chosen spirits did exist, and putting them in would have been hardly unreasonable. While the usage of Hyper Spirit Evolution would have benched the others anyway but it is all the more reason to give them time to shine before being reduced to cheerleaders for the last quarter of the show.
The four spirits used by the villains, later reclaimed by the heroes, are never used again. According to a relatively strong theory, the four kids encountered later in the series (Chaiki, Katsuharu, Teppei and Teruo) were intended as candidates for them. However, nothing of this is ever brought up.
The Beast Spirits become Out of Focus for the second half of the series, and don't even appear in the final episodes.
The Royal Knights. The group is far larger than the two Lucemon sent out to claim data. Baromon hints that they were well known, but this is never brought up again.
The Digital World's relationship with the Human World, something that was a key point for every other season, is never elaborated on.
The conflict between the Human-type and Beast-type Digimon which effectively started the events of the plot, according to the backstory for Cherubimon, is supposed to be an ongoing war by the time the series begins, yet it is never elaborated on aside from a single movie, and a non-serial one to boot.
Lucemon's origins. Unlike the villains of many previous incarnations, he's never given any sort of backstory or explanation as to where'd he come from, how he'd become evil, outside of an admittedly fitting sense of pride.
The Un-Twist: Lucemon was behind Cherubimon's actions. Given how the series decides to get mentioning him when revealing some of the backstory, it's no surprise that he plays at least some role.
Values Dissonance: Japanese divorce laws are very different from those in other countries, since a family is considered as 100% split when the parents divorce, and tend to effectively discourage, if not prohibit, contact between its former members. As a result, Koji's father hiding Koichi's existence, pretending that the twins' biological mother is dead, and effectively cutting off that side of the family comes off as a lot more of a jerk move overseas, when he probably was trying to spare Kouji from what he saw as needless suffering coming from not being easily able to meet his mother and brother per law. The American dub does try to lighten this up by softening some of his dialogue about Koji's birth mother and stepmother.
What Do You Mean, It's for Kids?: As usual in the franchise, Frontier could be definitely very hardcore for a show which was marketed to children in many countries. The new humanoid Digimon ensured the hand-to-hand was often savage, many villains were intimidating either for how they looked like or just for how shockingly dangerous they were, a Digidestined was killed (even if he got better) onscreen, some of the characters's backgrounds dealt with topics like bullying and parental divorce, and there was a perversely huge dose of Getting Crap Past the Radar. While Digimon Tamers might take the cake for the least family-friendly due to Konaka's foreign fandom and psychological touches, this series had actually little to envy.
A name was needed for Tommy/Tomoki's brother, who was simply called "big brother" in the Japanese version. Instead of pulling a Keenan Crier and making one up out of whole cloth, they chose "Yutaka". Not only is this an actual Japanese name, it's an anagram of Takuya, alluding to the deep friendship between him and Tomoki — Tomoki even calls Takuya "big brother".
Some people actually like that LordKnightmon was renamed Crusadermon and made female.
Zoe is a somewhat common name in Italy.
In the original version, Susanooomon only speaks with Takuya's voice. In the dub he speaks with Takuya and Kouji's voice.
In Spain, you will get only confused looks if you talk about Dynasmon, but they will instantly recognize him if you call him Dinocumon (or Dynocumon, or Dynokumon - it has never been revealed how exactly was it written, or why was it changed to begin with, but people seemed to like it).
WTH, Casting Agency?: Occasionally, the dub can get weird with this, likely as an aftereffect of Saban's demise and Disney's treatment of the franchise as a burden. Snimon sounds like Myotismon from Adventure, Cherubimon like Leomon from the same, and a Toucanmon and Mushroomon sound like they're channeling Kazu from Tamers and Davis from Adventure 02, respectively. Then there's Lucemon sounding like Izzy/Koushiro's dub voice.