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.
Badass Decay: Giroro. Although Giroro still the only one who seems genuinely militaristic in the platoon, and is not afraid to terminate with extreme prejudice when irritated. (Fortunately Keronians are Made of Iron.)
Viper. The first Viper is treated by the Keroro Platoon as a deadly serious threat, who easily captures one and brings the rest to their knees. From that point on, they seem to be a steadily decreasing threat, despite each successive Viper tending to 'upgrade' in some way.
Keroro in Tamama's eyes who laments that the legendary Gunsou san he's heard in stories is just a silly man child. Of course in that particular episode/chapter (episode 336 and chapter 140) the readers and Zoruru see a glimpse of Keroro we've never seen before .
Canon Sue: Pururu, who Keroro, Giroro, Dororo, Bariri, Joriri and Shurara all have or HAD a crush on. She appears in the third season (volume 13 in the manga), and it turns out that she's Keroro's childhood sweetheart and the school's idol. Even though she was NEVER mentioned before. She's also a member of the Garuru platoon. Natsumi likes her as well...
Natsumi is just weird on her own. She never EVER punishes Mois even though she has been as 'bad' as the others. Also, in one manga chapter, She claims the pool the Keroro platoon made as her own with a sign saiying: 'All aliens out'. But why is MOIS in the pool with her? There's a clear pattern here...
Tamama, with his immense jealousy of anyone who might steal Keroro from him, usually would have blown her to smithereens. The guy's had a 200+ episode-long rivalry with the only other female character who's remotely close to Keroro, so you'd think he'd be a little bit more pissed off about Pururu just waltzing in.
Subverted by the 6th season. Keroro is petrified when he sees Pururu's face without make up at the end of episode 266. It wasn't just a random joke either, as the opening of the 7th season references that episode directly. Episode 301 also clearly spelled out how only the old men, Bariri and Joriri, actually have a crush on her currently.
Creator's Pet: Natsumi Hinata, and all of her negative quirks in tow. She even shows up in episodes where she is least needed, while shoehorning her standard fiery condescending attitude.
Epileptic Trees: The fan theory that the Hinata Disappeared Dad is named 'Haru' in line with the family Theme Namingnote Natsumi = Summer, Fuyuki = Winter, Aki = Autumn, hence Spring = Haru is particularly popular.
I Am Not Shazam: Dub example—While making Ikinari dumplings at lightning speed on an assembly line, Angol Mois mentions feeling "just like I Love Lucy in that episode of I Love Lucy".
A real-world example is Keroro's name. Initially, there was confusion that caused some English readers to call him Sergeant Frog, since even though his name wasn't changed for the English release, that was the name the series got. Most people know better now, though.
Magnificent Bastard: One manga had her father try to force her into another private school, causing Fuyuki and the others to stand up to him. The end reveals that Dark Momoka planned the whole thing. She told her dad that she wanted to go to a private school so he could meet and gain respect for Fuyuki.
Narm: The third movie. Dark Keroro has nearly taken over the world and reveals his giant doomsday device. And what does it do? Slowly scoot along the landscape, plowing into things. Did I mention it moves REALLY SLOWLY?
And the goofy little dance Dark Keroro did to summon: didn't it crack you up?
The Second Movie. Where's The real "Deep Sea Princess"? It's was actually Natsumi all along wearing a some "Deep Sea Princess" gown with a Bare Your Midriff aspect.
Periphery Demographic: Some of the Asian countries with a substantial Keroro fanbase actually have mandatory national service, like Taiwan and Singapore. Do the math on that, and you've got a generation of otaku fanboys who have served in the army - rather like Keroro himself.
Replacement Scrappy: Joriri gets a lot of hate in fandom; his debut episode was suspiciously similar to a manga flashback starring Garuru, enough that fans immediately believed he was 'replacing' Giroro's older brother. Though the chapter was later adapted accurately, that first impression has not gone away, as he continues to show up and 'taint' the Chibi-Kero episodes.
Kululu is deliberately written as a Jerk Ass, and it's obvious everyone else is supposed to hate him, but it doesn't help that he does little of interest. More specifically, he's less of a character and more of a plot device used to add machines or science-goes-awry scenarios to the plot.
The anime's version of Dororo gets a lot of flak for seeming like a whiny, degenerate hermit compared to his more "speak softly but carry a big stick" portrayal in the manga. He's commonly demoted to being the Sixth Ranger (even though it's a Five-Man Band), not present in many plots involving the rest of the Keronians, and while he tries to be the Only Sane Man against their more wicked ideas, he's nowhere near as assertive as his manga counterpart and ends up the Butt Monkey a lot. What makes this all even worse is that his partner, Koyuki, is still shown as outgoing in the anime, and constantly interracting with her friends, which only draws attention to the fact that meanwhile, Dororo seems to be sulking in a corner somewhere.
Squick: The part in one of the earlier episodes where a half-drowned Momoka gets... CPR'd/molested just off-screen by a tentacle monster alien while the others look on with horrified looks on their faces — and then they all agree not to ever tell her that it had happened...
The dub takes this to the next level, stating that the "Life-o-puss" laid an egg in Momoka's stomach. When the creature zooms by in the scene after the eyecatch, Momoka asks if it planted something in her, which Fuyuki denies.
Kururu. Bath. Curry. That is all.
Stop Helping Me!: Pointing out the fact that Keroro made a doomsday device in front of the Hinatas at a time like this isn't really helping your cause, Mirara.
They Changed It, Now It Sucks: Purists tend to be incredibly vocal about Funimation's dub of the anime here, although in the minority, but even in its original form the anime attracts a lot of scorn from fans of the Manga. Common complaints are that the show was sanitized for younger viewers, reworked into a marketing gimmick, and the changing of Dororo from a Technical Pacifist who routinely stood up to his friends when they tried doing something wrong into a whiny Butt Monkey who barely even interacts with them.
Manga Keroro even breaks the fourth wall to lampshade this, telling everyone to clean up their act because they're going to be an anime.
Tropers on the Wild Mass Guessing page have gone even further, suggesting that the anime is a cheap money-making scheme orchestrated by the Keronians.
Unpopular Popular Character: Kururu/Kululu. In universe, he is the least popular among the platoon (not counting Dororo who everyone just forgets) with his toy selling worst out of them all. In real life, he is frequently at the top of peoples favorite character list, usually in front of or behind Giroro.
Was It All a Lie?: Anything past the touching Season 1 finale can be viewed as this, and makes it apparent that Keroro will never have any form of happiness.
"Weird Al" Effect: The anime commonly includes Shout Outs to older works to entertain some of its older audiences, so naturally for many younger viewers, it's often the first they've ever heard of certain things.
Lampshaded by the Dub, in which the narrator tells people to search for Space Sheriff Gavan on Youtube. Interestingly, that show actually was shown in America, but it's highly likely that most viewers never saw it.
The Woobie: Dororo. He takes a lot of abuse, especially in the early seasons of the anime anime, which just love sending him back into his Corner of Woe every chance it gets.
Fuyuki to a lesser extent. Before Keroro came into his life, he had no friends and nobody joined him in the Paranormal club. This really shows in the Valentine's Day episode.
Woolseyism: The FUNi dub looks set to be full of these — Natsumi using her leek on Giroro's boobytraps has already been turned into an outright reference to the Loituma Girl.
As mentioned previously though, the FUNi dub doesn't really know whether it wants to be this or a Gag Dub. It's sort of both.