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.
Being a middle schooler doesn't protect Fuuka from being bashed heavily by Sana/Akito fangirls. Especially with how the anime handled her eventual relationship with Akito and their breakup. It doesn't help that she shares some similar aspects with Sana, even physically looking just like her. She is also seen as a Replacement Scrappy because of the implications regarding to why Akito is dating her.
Naozumi isn't exactly liked, either.
Ear Worm: A few songs, but particularly "Ultra Relax".
Friendly Fandoms: People often compare Sana and Akito's relationship to Mikan and Natsume's on Gakuen Alice. Kodocha is also referred affectively in some circles as "Marmalade Boy on Crack", given the similarities in between Sana and Akito's physical looks with Miki and Yuu.
Toy Ship: Remember that Sana and Akito were introduced as 11-to-12 years old at the start of the series, and (while discounting the epilogue of the manga) they are still in Middle School (age 12-to-13) through the end of the series.
Ugly Cute: Babbit is a creature that looks like a mix of a bat and a rabbit (hence its name), it can be annoying (especially when the animators deform it more as a caricature of itself) but it has its charm.
The manga was published by shoujo demographic magazine, Ribon. However, it has some dark but dramatic moments and topics that make you doubt if this was a series directed to little girls and preteens. This starts with the scene of Akito asking to Sana to kill him. The manga also deals with Teen Pregnancy and rape, as it is implied with Sana's biological mom being impregnated at 13 years old by her uncle, Abusive Parents (implied that Tsuyoshi's dad was not only an alcoholic but also that he abused his mother), Suicide, and using Sex as Rite-of-Passage as a plot device for Sana and Akito in a very misguided attempt to stay together.
Miho Obana does not hide the fact that she wanted to work shoujo manga with darker tones. "The Mansion of Water" movie (being filmed in one of the arcs of the series and later adapted into an one shot) deals with dark and horror, something that she would later expand on other mature works as Partner and Honey Bitter.
The anime, while taking a different route from the manga's original direction, also has its own moments. The last arc looks like a distillated version of a later arc on the manga, but it shares its own drama- quite different compared to what the first episodes looked like.