These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Ben 10: Alien Force
Accidental Innuendo: The show has Kevin uttering the line "I don't know if I can pound your Grandma!" He means he's unsure he can fight her ...but still.
Gwen's and Kevin's crushes on each other are just their way of repressing that they're both attracted to Ben. Both attractions are at least partially taboo — Gwen's because Ben's her cousin, Kevin's because Ben's a guy — so the only solution is to try to force themselves to fall (however improperly) for each other. Because they're both lying to themselves and each other, they're both getting their signals returned, which is encouraging the charade. This explains why Gwen tells Kevin to ask her out without wanting him to, why Kevin tries to make himself appear attracted to Gwen without making a move, and why they both show Ben so much more subtle and genuine affection.
Angst? What Angst?: Despite Grandpa Max apparently dying in Season 1, Ben and Gwen's fathers don't seem to be in particular mourning over him. To be fair, they're only seen in one episode apiece, but one such episode ("What Are Little Girls Made Of") does reference what happened to Max quite a bit.
Also, right after he seemed to die, Ben and Gwen immediately composed themselves. No tears, no anguish, no nothing.
Badass Decay: Kevin and Vilgax suffer a lot of this. The general consensus is that Kevin got better in Season 3 (something that is even lampshaded in "Time Heals"), but Vilgax (ironically only featuring in that very season) never recaptured his former glory.
Complete Monster: Ragnarok. He destroys suns for a living, not caring if planets in the system support life or not. And all for money at that. And apparently, he likes doing it, since he waited the entire time his sun destroyer was sealed away trying to find a way to recover it instead of moving on to something less omnicidal and treats killing Kevin's father like no big deal.
Crowning Moment Of Awesome: Rath diving into the giant Trigonesque alien to rescue the Tiffin and escaping by punching the guys teeth out from the inside and threatening to "knit [his] organs into a sweater" if he tries to retaliate.
Damsel Scrappy: Julie is perceived like this by some fans, but it's mostly an undeserved reputation caused by What Measure Is a Non-Badass?; she actually got kidnapped only thrice over the course of five season, which is fairly low as far as Super-Heroes' Love Interests go, and, by the time she got Ship, she was perfectly capable of defending herself of her own.
Big Chill also is fairly popular amongst fans for his cool design, similarities to Ghostfreak and good voice;
Albedo, who rarely appears, is surpisingly popular among the fans too, as mentioned previously.
And let's not forget time-travelling the DoctorExpy Paradox, whose even most of the Alien Force haters actually love.
Fangirls and the furry fandom consider Argit as this.
Reinrassic III; granted, he is an important character, but he only had three appearances in the whole show. Yet you would be surprised to see how many fanart there are of him.
Evil Is Sexy: Still Charmcaster. Ben and Kevin even lampshade it in the episode she appears in.
Ben: Was she cute?
Kevin: More than cute; hot!
Fan Dumb / Hate Dumb: While some of the criticism to Alien Force are justified, the hate toward the show from original series fans can go to the ridiculously extreme occasionnally. Here are most noteworthy examples:
When the trailers were first shown, fans were already complaining about the new aliens "looking weak". And their powers had not been revealed yet back then.
And let's not even talk about the ridiculous amont of Hate DumbDwayne McDuffie suffered from some of the original series' fans. Some of them have even gone as far as denying any of his work was ever good (remember this guy has worked on stuff such as the animated version of Justice League, created Static and made several of DC Original animated movies), and cheered up after his death.
Fan-Preferred Couple: If you haven't already noticed that there's a large fanbase who hates Kevin/Gwen and wants Ben/Gwen, this is probably the first time you've ever visited Tv Tropes (though oddly, Ben/Julie isn't quite as reviled after Julie got Rescued from the Scrappy Heap.)
Genius Bonus: The Tiffin. If you know what the term means then it's no surprise that he's going to be eaten.
Growing the Beard: Season 1 didn't really have a clear story arc and included many fillers. Season 2 improved things a bit, continuing previous episodes, giving a slightly clearer progression, rescuing Julie from the Scrappy Heap and actually concluding to a real finale. Sadly, season 3 didn't work that well.
Misblamed: When many of fans of the original Ben 10 complained about the questionable changes, they often blamed Man Of Action for it. Despite the fact Man of Action has little to nothing to do with this show or any other beside the original except serving as executive producers.
Also, Rath's "Lemme tell yah something" catchphrase have become quite popular.
Narm: Vilgax's so-called ultimate form? A giant green octopus. Sad because it could have been a great scene if not for limited efforts on the design and a fight that is not even seen onscreen.
Never Live It Down: While Vilgax did suffer Diminishing Villain Threat in Alien Force, he still had his share of Bad Ass moments, including having a Curb-Stomp Battle with all the Plumbers Helpers and Max, defeating several of Ben's most powerful alien forms (Jetray, Big Chill, Humongousaur and Chromastone) one after the others, destroying a whole ship of his own to free Ghostfreak, having a decent fight with Ghostfreak (which he won) and actually getting the Omnitrix in the finale, which led to the show's Darkest Hour . However, the only things most fans retain about his Alien Force incarnation are that he was following a Galactic Code (even through both the series and Word of God shew he did it only out of Pragmatic Villainy) and that he got defeated by Diamondhead, who couldn't even scratch him before.
There are many who don't like the Anodite species as a whole, due to what is felt to be a pointless, as well as sometimes incredibly confusing Retcon, with the representatives of the species not being very well-regarded either.
Seasonal Rot: The third and final season seems to be the least popular overall, mainly due to Vilgax's Diminishing Villain Threat; even after it (mostly) came back in the Grand Finale, the actual fight between him and Ben was seriously disappointing, despite the rest of the finale being awesome. Not that it's entirely hated though, as it did introduce us to RATH!!!
Strangled by the Red String: Gwen and Kevin — The ship is dropped on the viewer with absolutely no set-up, no basis, no rationality in the very first episode. Kevin almost wrecking his precious car to tell Ben not to be rude to Gwen could be attributed to his Badass Decay, but "I'll follow you anywhere"? Were the writers afraid that if they took the time for some actual development, the show might be canceled before their new favorite couple actually got together?
And even what they did of this plot didn't amount to much. For a series called Alien Force, the only major things thing they did was fight the Highbreed at the end of season 2, get their asses handed to them by Vilgax at the beginning of season 3 and then got sent off to the Plumber Academy after "Above and Beyond". One must wonder what the point to this plot was when pretty much everything important amounted to Ben, Gwen and Kevin (and Max after season 2)doing all the work.
Some people complained about things that are established, but are just dropped, or otherwise not elaborated upon (Why Ben decided to remove the Omnitrix, which could have tied into him wondering if his job as a hero is worth it, Kevin being Easily Forgiven and almost entirely trusted, among a few other things.).
They Changed It, Now It Sucks: Everything has been blamed to be this at least once, from Gwen's alien origins to the Omnitrix's new shape, without forgetting Vilgax's new design, the characters' change of personnality (especially Kevin), and the list goes one.
Ugly Cute: Argit. There are some people out there (mostly fangirls and furries) who adore him.
Unpopular Popular Character: Argit again. In-universe, he is despised by almost everyone, and seems to have people who want him dead in every part of the galaxy. On the fandom side, on the other hand, you would be surprised to see how many fans love him. Even Derrick J. Wyatt, the one in charge of Ben 10: Omniverse, admitted he thought Argit was the best thing that came out of the Alien Force/Ultimate Alien era.