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: Shugo Chara!
Broken Base: A majority of the fangirls HATE each other depending on who they're shipping. They usually complain that the other ship's fanbase is immature (it's a case of the pot calling the kettle black).
Family-Unfriendly Aesop: In one episode of Dokki Doki (which is part of Party), Utau storms out of a recording session because she didn't want to change her singing style. After a brief battle with an X-Character however, she is "inspired" to return and record her new song in the style they wanted. This probably wasn't intentional, but it nonetheless implies that 1) you need to do what your told, even if you don't like it and 2) you need to accept "changes" to a product (e.g., ramen in one scene) even if you don't like them, and even if the company (or whatever) that makes it "sells its pride" (as Utau put it). The episode probably meant to give viewers the "try new things" message, but ended coming out the wrong way.
Freud Was Right: Amu possesses the Humpty Lock, and Ikuto has the Dumpty Key. Amu pretty much summed it up...
"Why does he have a key that fits my lock?"
And long before things get to that point, we see Amu asking Ikuto, "Why do you want my eggs?"... after a scuffle that left her lying on her back, and him on his hands and knees, straddling her. Five minutes later, she is covered in white goo.
Ho Yay: Let's see, there's Utau's Tsundere-ish behavior towards Amu, Rima's possessiveness of Amu, and on the flip side, Tadase and Ikuto have some subtext going on, especially when Tadase starts ranting about how Ikuto only befriended him in order to steal his key.
I Knew It: A few fans probably connected that Tsukasa is the future Tadase.... or is he?
Internet Backdraft: Amu and Ikuto's relationship. Is Ikuto a pedophile that has such a lust for Amu so Squick inducing? Or are all the cries about their age difference and all are just a Double Standard against men going in effect, and that Ikuto's love for Amu is just as legitimate and natural as any other couple? Regardless of what you think, watch out for the Flame War.
Memetic Mutation: AMV makers got obsessed with Ikuto's pelvic thrusts in the second season's second OP.
That probably wasn't a good idea on Satelight's part...
"CHARA-SU!note A combination of "chara" and "osu"; the latter is an informal greeting Minna kyou mo Shugo (Shugo) genki?"
"HISSATSU!!...Merry Merry!" Explanation It looks as though Yaya is going to unleash an unbelievably powerful attack...then out comes a baby mobile (in accordance with her "baby" theme) that sprays sleeping gas everywhere, putting the X-charas to sleep...and her along with them.
"GO! GO! LITTLE DUCKIES!" Explanation An attack that consists of a "salvo" of cute rubber duckies. Another "ineffective" attack...until they become "sexy, cute" black rubber duckies...
Ron the Death Eater: 2/3 of the fanbase seem to antagonize Tadase for no other reason than being in the way of Amu/Ikuto. But, in order to keep it PG, most Shugo Chara! fanfictions will use this to get rid of him.
Pair the Spares: Some people also like to pair him with Yaya. That is, if she isn't already paired with Kairi.
Ship-to-Ship Combat: Tadase and Ikuto for Amu. And much like any shojo series with romance, sometimes it can get pretty serious. Insults being thrown left to right, it's pretty messy.
The Scrappy: Tadase gets this so bad by Ikuto fangirls, it's not even funny. Also Rikka in the third season of the anime.
They Changed It, Now It Sucks: Some of the fans now see the new season Shugo Chara Party as this. Especially on Crunchyroll, where the show is simulcast. Many of them positively loathe the variety show elements (Party is hosted by Shugo Chara! Egg in live-action segments, and features a short anime segment with just the Charas, in addition to the main segment).