Broken Base: Every sequel following the original series only divided the fanbase further and further. By the end of Omniverse, the division could rival the one for Star Wars.
The different tones of each series naturally caused a huge divide on which was better. There are those who liked the original series and Omniverse for their charm and felt that the more serious tone of Alien Force and Ultimate Alien was unfitting for the franchise. Those for the more mature series feel that the mature tone was natural since the audience was growing up with the franchise, gave more opportunity for Character Development, and that the later attempts to change the tone back to the original series hampered the franchise and was ultimately what caused the reboot.
The series being rebooted. Some fans were happy, saying the show was best when the main characters were 10-year-old Ben, Gwen, and Grandpa Max, that the franchise had suffered so many Retcons that a reboot was needed, and that the reboot would likely bring over things people liked about the sequels in a more organic way. Others accused the reboot of being an attempt to draw more cash out of what was already a large Cash Cow Franchise (merchandise has sold over four billions at least), and hated the art-style, saying Ben looked a lot younger than 10 and more cartoonish. There's also many who wanted a sequel to Omniverse instead of a reboot, especially with the former ending's Sequel Hook that sounded really promising.
Complete Monster: Vilgax, the Overarching Villain of the entire franchise and by far Ben Tennyson's most personal enemy, is a psychopathic galactic warlord. After his evil caused him to be banished from his homeworld, Vilgax went on to assault and conquer numerous planets across the cosmos, outright annihilating the entire planet of Petropia and its billions of inhabitants to make an example of those who resist him. After discovering the existence of the Omnitrix, Vilgax becomes obsessed with possessing the gadget to modify it for his own gain, and spends much of the first series sending wave after wave of his forces to Earth solely to kill Ben Tennyson, a 10-year-old-boy, and steal the watch from him, later trying to viciously saw the boy's arm off and blow up the entire Earth for the trouble it has caused him. In Alien Force, Vilgax builds up his image as a law-abiding citizen by "legally" conquering 10 planets, though ultimately ditches this persona and tries to use the Omnitrix to lead a full-scale war against all who would stand against him. In Ultimate Alien, Vilgax becomes the herald for Diagon before betraying and killing his master and stealing his power, hoping to use it to mind control all life in the universe. In the final series, Omniverse, Vilgax, though at first seeming to have decayed in his evil, pulls out perhaps the most wicked scheme in the entire franchise, using a Chronosapien Time Bomb to wipe out all dimensions and timelines, his own included, to leave only a single, more submissive universe for him to conquer and rule. Vilgax was a purely evil madman who would kill first billions, then insurmountable trillions, just to satisfy his raging desire to be the most powerful being in the universe, and stands out as the most iconic villain in Ben 10 for this very reason.
Evil Is Sexy: Charmcaster. Which of her character designs is sexiest is up to debate, though.
Fanfic Fuel: Each alien in the Omnitrix belongs to an alien species with its own planet and culture, leaving huge possibilities to create various Original Characters. The Omnitrix is also stated to have at the very least 10,000 aliens (more than 1000,000,000 after Alien Force), meaning fans have a large gap available to imagine and create their own aliens.
Fan-Preferred Couple: Go on any fan art site or shipping thread and it quickly becomes apparent that Ben/Gwen is the fandom's OTP by a landslide. Mainly due to their constant bickering and Ship Tease moments, along with the fact that Gwen was originally planned to be one of Ben's classmates. The decision to make them cousins instead did nothing to deter the shippers.
Memetic Troll: The Omnitrix is often seen by fans as this thanks to both the hints of it being sentient and the Flanderization of its tendency to give Ben the wrong alien.
It's a franchise about a Kid Hero with a watch transforming him into Alien Super-Heroes, fighting a Galactic ConquerorCthulhumanoid, with the help of his Magical Girl cousin and his Hawaiian-shirt wearing grandfather who used to be part of a secret organization called the Plumbers. It doesn't prevent the franchise from making some genuinely good stories and nightmarish villains.
One True Pairing: The aforementioned Ben/Gwen, due to what became a majorwriting fumble. Gwen was originally planned to be one of Ben's classmates, but the writers found it difficult to justify having her tag along with him on his adventures. So they made her his cousin, except the 'damage' had already been done due to their constant bickering being perceived as Belligerent Sexual Tension, in addition to what appeared to be Ship Tease. Not even officially pairing them off with other characters changed the shippers minds. Thus, Ben/Gwen has remained THE overall preferred fan pairing.
So Okay, It's Average: The franchise as a whole is usually seen as this by non-fans. While it has a lot going for it such as good fight choreography, creative aliens, and a lot of good talent (writers and voice actors alike), it also suffers from inconsistent writing, fairly basic characterization, and rarely goes very far into worldbuilding the universe the characters live in, with a lot of it contradicted across the shows due to the different creative teams at the helm. Even some actual fans will admit that while they love the franchise, they do wish it had been more.
Tainted by the Preview: The Comic-con preview clip of the reboot gained a large amount of flak on its reveal. Points of interest include Gwen and Grandpa Max having different voice actors, Diamondhead having different powers, the more simplistic designs, and the background characters having the same problem as in The Powerpuff Girls (2016). note They look like they belong in a completely different cartoon, only here thanks to the trace amounts of Animesque, it's much more jarring.
Angst? What Angst?: Camille from ''Big Fat Alien Wedding" seems to get over her parents turning against her, trying to kill her fiance, and then promptly getting turned into a statue courtesy of Heatblast rather quickly.
Lucky Girl, who is ironically one of the leads in disguise. Her nemesis Charmcaster qualifies on the villain side of things.
Naturally, some of Ben's aliens are more popular than others, but the one who takes the cake is Heatblast, who is the most popular and iconic alien form in the entire franchise. He is so popular that an Ultimate Alien DVD was titled "The Return of Heatblast".
XLR8 is very popular with the fans, which makes it extra baffling that they went and made Fasttrack.
Buzzshock and Arcticguana, two of Ben 10,000's alien forms, are surprisingly rather popular among the fanbase. They both showed up in Ultimate Alien's intro (but never in the series itself), before finally being formally unlocked in Omniverse.
Ripjaws has only appeared 16 times in the entire Ben 10 franchise, but he is popular with fans in addition to getting lots of Fan art and YouTube videos.
Diamondhead, despite having a lesser amount of appearances (16 episodes) in the original series of all the ten original aliens, is popular amongst fans thanks to his unique design, voice, and cool powers. He was so popular that he made a return in the third season of Alien Force, temporarily replacing Chromastone and being one of the few aliens to defeat Vilgax.
Sixsix, a minor villain thats introduced early on in the series, quickly became beloved by fans due to essentially being the Ben 10-verse's equivalent of Boba Fett. He was so popular that not only was he brought back later on in Omniverse, which also introduced his family made up of his equally badass brother and two sisters and changed him from a one-off mook to a series mainstay.
Evil Is Cool: This show is a master at creating awesome bad guys. Vilgax, Dr. Animo, Kevin, Ghostfreak, Hex, Charmcaster, the Forever Knights (especially the Forever King), Sixsix and his family, and nearly all the other villains (with the exception of Sublimino, who's a No Respect Guy even among his teammates) who made up the Negative 10 could count.
One complaint about the sequels is how redundant some of Ben's aliens can be. The original series has many aliens who have unique abilities, but some have similar advantages to one alien that leave another alien functionally redundant (particularly Benvicktor/Frankenstrike, who on top of having super strength, has lightning abilities), indicating that there were ability and redundancy problems even before the sequels.
A lesser example, but one of the major criticisms made towards Alien Force was (amongst other things) about the Plumbers being inexplicably active again despite it being stated to have disbanded in the original series. "Big Fat Alien Wedding", while not explicitly stating the Plumbers were still active, made mention of a war between them and the Sludgepuppies that was apparently still going on (the whole point of the episode was to end it), suggesting at least some of them kept fighting.
Gwen is apparently popular enough in Southeast Asia that Cartoon Network Asia gave her a website and a good amount of merchandise. A spin-off show about her was even considered at several different points in time in answer to her popularity, with her Omniverse episodes in particular serving as a Poorly Disguised Pilot.
The franchise is also very popular in Denmark, where many stores hold entire sections of Ben 10 merchandise.
The franchise has a strong following in Latin America as well, seeing as at one point nearly all of its schedule was devoted to showing Ben 10 episodes.
Tetrax's admonishment of Ben, telling him that he fails to consider the consequences of his actions until they blow up in his face, takes on a new light when it's revealed later on that Tetrax gave Vilgax a crystal that he used to destroy Petropia, which is an action he came to completely regret.
In "The Unnaturals" after Cash and JT win the go ahead run against a robot team, oblivious to the fact Ben was helping them, Cash goes "Hey, hey, hey.", referring to Fat Albert and the Cosby Kids. This is a lot harder to listen to after Bill Cosby's conviction for rape, as Fat Albert was frequently used to mock him.
Heartwarming in Hindsight: "Ken 10" had Ben's future self Ben 10,000 name his son Ken. The sequel series Ben 10: Alien Force revealed that Ben's cousin Gwen had a brother also named Ken, which makes it rather sweet when you realize that Ben may have named his son Ken after his cousin Ken.
Remember that ending in "Gwen 10" where the Omnitrix ultimately ended up on Grandpa Max? It is revealed several episodes later by some alien named Xylene, a person from Max's past, that the Omnitrix was originally for Max. But due to the fact that Ben's DNA was close to his, Ben was the one who discovered the Omnitrix and it ended up on him.
In the end of "Kevin 11", Ben offers Kevin to join him and become allies. Soundfamiliar?
In "The Big Tick", while trying to figure out what Cannonbolt does, Ben lists powers of aliens he will receive in the future: breathing firenote Ultimate Big Chill in Ben 10: Ultimate Alien is capable of exhaling icy fire., firing freeze rays from his eyesnote The episode aired long before Ben was confirmed to ever get access to an alien form with ice powers, such as Articguana or Big Chill, and magnetic blastsnote Lodestar, who debuted in Ben 10: Alien Force, has magnetic powers.
Vilgax is an expy of Darkseid. In the original series and Omniverse, he was voiced by Steve Blum. Guess who voiced the actual Darkseid in Justice League: War.
Ben's dislike of the Super Alien Hero Buddies for being bastardizations of his own alien forms in "Super Alien Hero Buddy Adventures" is funnier to look back on in light of the polarization and controversy respectively caused by Omniverse and the reboot trying to be Denser and Wackier than Alien Force and Ultimate Alien.
The huge success this series had and the fact many fans are only loyal to it has caused many people (both fans of the sequels and people who don't particularly like the franchise) to be more critical toward it and point out its flaws.
Man of Action Studios used to be considered Only the Creator Does It Right for the franchise and the fact they only worked on the original series was used as a justification for not liking the sequels. Since then, their work on very unpopular Marvel cartoons have caused many people to notice flaws in their writing that already existed in the original series.
Tetrax being a Petrosapiens (AKA the same species as Diamondhead) was a twist in his first episode. By now, it's nearly impossible to not be spoiled about it in discussions with the fandom, seeing how this became a major plot point in both the movie and a season three episode of Alien Force. Also, the fact that his home planet was destroyed by Vilgax is this trope to a lesser extent.
Vilgax destroying Tetrax's home planet, killing hundreds of innocent lives for no reason other than to test his brand new toy.
When Ghostfreak wanted to plunge Earth into darkness, the result of which is shown to be horrifying mutations of all living things. He also crossed the line in "Ghostfreaked Out" when he threatened to kill a possessed Gwen if Ben did not surrender, fully aware that Ben is only 10 years old.
Also note, that was just a side effect of his plan that he didn't even care about.
Zombozo gets his by kidnapping a 10 year old girl and draining her of her energy until she's nothing but a shriveled husk of her former self. The fact that he speaks of it so lightly that it's implied it's not the first time.
As if it wasn't clear that the Forever King is more evil than Enoch, he deliberately leaves Enoch trapped forever in his Lotus-Eater Machine just to spite him for not getting the Omnitrix.
My Real Daddy: For fans who prefer this series yet are disappointed by Man of Action's later reboot, the head writer duo of Thomas Pugsley and Greg Klein get the "Real Daddy" position over Man of Action.
Older Than They Think: There really was a secret organization called the Plumbers - Richard Nixon's private police who did the illegal surveillance of the Watergate hotel.
Xylene. She is an alien Action Girl who has cool powers and a unique design along with a big part in Ben's origin since she was the one who brought the Omnitrix to Earth and had a past relationship with Max. She appeared for exactly one episode and never appeared again. Even in the sequels, all she got is a very brief mention in a dialogue.
The Galactic Enforcers only show up in one episode along with an appearance in the future episode despite having included Ben in their ranks as a honorary member.
At the end of the third season, Ben gets three new aliens, each based off a different monster. A werewolf, a mummy, and Frankenstein's monster. Despite all of them being pretty cool, they don't appear at all for the rest of the series and it isn't until an alternate timeline episode of Ultimate Alien that any of them reappear. They aren't even properly named via being referred to as "Ben*monster name*" in the credits. It took until Omniverse for them to even be given more appearances and proper names.
Viewer Gender Confusion: Ripjaws (who is an alien in Bens Omnitrix) is part deep-sea creature and merMAID. While he might seem like a female, he's actually a male... though this could be a case of people calling something by a gender-specific title despite it not being that gender, similar to how female peacocks are often called "peacocks," by the general public, despite the appropriate name for a female of that species being "Peahen."