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.
Also the OP, from the same group (Round Table ft. Nino) that did the Welcome to the NHK! OP.
The Dark Chi Theme. It that makes you feel not just the suspense of action sequences, but truly the emotion behind them too. Not only is it beautifully composed, but the musical flow typically goes PERFECTLY with whatever scene it's set against. Be it Chii's defending herself against Dragonfly or Hedeki rushing into aHopeless Bossfightin order to play an RPG together with her. It's never inappropriate when used and it's always the perfect choice for the scene.
Yasashisa no Shouzou; a soft, slow piano and violin tune that manages to be sad, happy and sentimental all at the same time. Ususally popping up in soft, emotional or sentimental scenes, the song is a Tear Jerker on it's own.
Ear Worm: Doo doo do doo yeah. Doo doo do doo yeah. Doo doo do doo Let me be with you.
Fandom Berserk Button: Referring to Freya as "Dark Chi(i)" (though she was called that a lot before her actual name was revealed) gets some fans riled up. Also, the Chi/Chii debate can be an issue for some.
Freud Was Right: Freya has an Electra Complex. She fell in love with her father/creator, choosing him as "the someone just for her," and felt very sad seeing him spending time with her mother.
Les Yay: With Chi's other self, her twin sister Freya.
Moe: Oh, Chi...So sweet and innocent and adorable.
Squick: Your tolerance for this series depends on how seriously you think about the Unfortunate Implications discussed below. Some would call the premise 'disturbing'.
Chi looks sixteen, acts like she's eight years old, and has No Nudity Taboo. It makes every moment of her nudity that much more awkward.
Chi's relationship with Hideki is played up as being "pure" and "innocent", but Chi is still heavily dependent on Hideki for many things and is as naive as a child. More often than not, they're more like Fatherand Daughter than anything else.
A lot of Chi's clothes come from Ms. Hibiya because Chi/Elda and Freya were built by her husband to be their surrogate daughters since she was infertile.
Minoru is 12, may not have parents, and is such a genius he knows how to build persocoms. Yuzuki is in a super modest maid's costume but his other four persocoms...are all busty women dressed in maid outfits so fetish they're essentially in bras or corsets with garter belts and a frilly apron. Minoru tells Hidelki they're "only computers" and not to get so aroused, nor that they put out. note He probably does it for the reaction he gets out of guys like Hideki. He may have considered the whole thing an elaborate prank.
All Most Persocoms are attractive women note (male models can be seen here and there, and are mentioned more than once with one even being a plot critical character.) designed to be all-purpose servants. The pronounced gender-line probably has to do with the fact that, in Japan, robots are seen as inherently feminine, unless noted otherwise - because they're created to serve.
Although Hideki is clueless, and his landlady is not, nobody offers to help get Chii's programming sorted out so she doesn't ever progress beyond talking/reasoning like a little girl. Yet she falls in love and Hideki reciprocates.
There's a lot of subtext that amounts to "If you can't have children, you're inherently less of a person for it".