Considering that Nasa is a studious young man who's rather dense in social situations, is his expertly-worded (if ultimately futile) attempt to win over Chitose proof of how well he's studied rhetoric, or just him winging it and happening to get lucky?
How good is Nasa's relationship with his parents? He seems to love them, even if he's bitter about their decision to give him his Embarrassing First Name, but he doesn't keep in very close contact with them, only telling them about marrying Tsukasa when he realizes that he'll need his parents as a guarantor if he moves into a new apartment. His decision to move out of the house because his parents were worried about his overworking himself is either due to being considerate of their feelings or because he doesn't actually care how they feel and simply wants to do as he pleases without their objections. In either case, it's worth noting that Nasa's parents' decision to move away was not due to any of Nasa's actions.
Angst? What Angst?: Nasa isn't at all upset when his apartment burns down while he and Tsukasa are seeing his parents, and his first thought is to ask if anyone got hurt. It's largely justified, since he had prepared for something like that happening, he and Tsukasa find a place to stay before long and Tsukasa notices that he's the sort of person who puts others first, but one would think that he'd at least get somewhat upset.
Arc Fatigue: Despite the series managing to cut through the Will They or Won't They? by having the main couple get together at the start of the series, it can suffer from this if you're not into the whole "building a relationship to justify the marriage" angle that's played in a (mostly) Slice of Life fashion.
In Chapter 30, Nasa's apartment building burns down, resulting in him and Tsukasa planning on moving into a new apartment together. In Chapter 153, after the end of the flashback arc, Nasa reveals that the landlord had contacted him while Tsukasa was away, revealing that their new apartment was ready, but Nasa had to cancel the lease because he didn't know when or if Tsukasa would return. The two of them eventually agree that they like staying with the Arisugawas. As a result, the idea of moving gets tabled, possibly permanently.
The series occasionally drops hints at who exactly Tsukasa is, but it takes more than 140 chapters to get around to revealing Tsukasa's full backstory.
Awesome Music: No one expected this rom-com to have such an absolute banger of an opening in Koi no Uta. Who knew that Akari Kitou could sing that fast?
Nasa has a lot of fans for his Memetic Badass qualities, particularly how he doesn't waste any time getting together with the girl he likes, survives getting hit by a Truck(albeit with Tsukasa's help) and is exceptionally intelligent. Detractors, however, consider him an overly perfect self-insert who never struggles with anything, dislike how he often acts like a grade schooler who's never met a girl before (despite having long been friends with the Arisugawa sisters), and/or find that his Covert Pervert tendencies more disturbing than the series presents them as.
Depending on who you ask, Tsukasa is either an enigmatic and adorable wife with a potentially interesting backstory or someone who lacks any agency of her own and lets her life revolve around her husband.
Early on in the series, Nasa sometimes worries about Tsukasa suddenly disappearing from his life, which isn't played all that seriously. In Chapter 142, he wakes up and finds Tsukasa gone, with her bedding neatly folded, which makes his fears seem more justified in hindsight.
Tsukasa's dilemma shares a number of details with two girls in other media, one of whom even has a similar backstory as her:
In Shiren the Wanderer 3, Kaguya was the basis for The Tale of the Bamboo Cutter. Her father was killed by the greed of another person, she was caught in a fire after she was betrayed by someone she trusted, and an "elixir" of Hourai allowed her to survive the resulting burns but also put her in a thousand-year coma. Unlike Tsukasa, Kaguya never got to peacefully spend her days with the person she all but explicitly loved, but her death at the end of the game implies that they finally get to reunite in the afterlife.
In A Certain Magical Index: Miracle of Endymion, Ladylee Tangleroad also gained immortality when she consumed something offered by another (ambrosia from a knight of the Crusades as thanks for saving his life), and grew increasingly suicidal after living a thousand years. Unlike Tsukasa, Ladylee's morals are far more selfish, such that she's all too willing to let innocent bystanders be killed by her suicide schemes. Had she succeeded in the movie's climax, everyone else in the world would've been dead, and she could've potentially survived it anyway.
When Aya sees Tsukasa in the bath, she ends up gushing over how pretty Tsukasa is, even if she ends up being embarrassed about the analogies she chooses. Tsukasa gets a bit jealous over Nasa thinking that Aya is pretty, but it's worth noting that Tsukasa was the one who brought it up in the first place, implying that she also thinks Aya's pretty.
Chitose immediately becomes hostile toward Nasa after realizing that he's married her sister Tsukasa, believing that he's unworthy of being with her when he doesn't know of her past, and that she deserves to stay by Tsukasa's side, since Tsukasa also saved Chitose's life. When she screams, "I'm the one who needs to marry Nee-sama!"note In the dub, she refers to Tsukasa by name in this line instead of calling her "Big Sister.", it's hard to tell whether she thinks of herself as Tsukasa's (unrelated) little sister or her lover.
When Chitose explains the "Narita Divorce" trend, the anime has Charlotte and Aurora act as the married couple in question.
Memetic Badass: Nasa got this reputation due to the fact that he actually survived Truck-kun(albeit because Tsukasa protected him) and instead of getting sent to another world, got married to his Love Interest by the end of the first episode and without wasting time on Will They or Won't They? or a harem. His intelligence is also a reason for his reputation, so much so that in-universe, Kaname compares him to Tony Stark.
Memetic Mutation: Coupled with the thing about Truck-kun above, fans like to joke that Nasa did get killed by Truck-kun, but the other world he was sent to was one where he survived and Tsukasa eventually married him.
Nasa gave the Arisugawa sisters some business advice to help keep their family bathhouse open despite only being in middle school at the time. His advice worked, which is why they feel indebted to him.
Nasa immediately understands how the display case is keeping Tsukasa's moon rock preserved just by looking at it, and quickly fixes it when Charlotte breaks it.
Tsukasa pretends to have fallen for Chitose's fake photo of Nasa and Charlotte, then drags him away for a serious discussion. This kills two birds with one stone- getting a pretext to get Nasa out of the room and convincing Chitose that her plan had worked.
The Scrappy: Chitose, and to a lesser extent, her maids too, for constantly getting in the way of Nasa and Tsukasa and trying to break them apart in order to have her adoptive sister all to herself. To say that the fans enjoy any chapters devoid of her presence is an understatement.
Tear Jerker: Combined with Nightmare Fuel, chapter 148-151 is a flashback to Tsukasa's early life. Specifically, how she attained immortality due to her father's misguided attempt to save her life, how the villagers immediately turned on and hunted her down, how she had to kill them all, and all the despair that came with immortality.
Values Dissonance: Some Western viewers are uncomfortable with the idea of Tsukasa, a 16-year-old, marrying Nasa (who, while close to her age at 18, is rather young for marriage), but in Japan, it's legally possible to get married this young with the couple's parents' permission. Even so, some people point out that this doesn't really make it better, since Tsukasa is implied to have put down fake names for her parents.
The animated film:
Ass Pull: Shortly after the flies go into orbit. Something goes wrong inside the rocket's control panel thanks to a wire that had gotten loose. The flies fix it, using an olive to insert it back in without getting burned. All fine and good, except we're never shown where the olive came from.
Audience-Alienating Premise: Flies sneaking aboard the flight of Apollo 11. With that rather uninteresting synopsis coupled with the hideous character designs, it can be presumed most people avoided it.
Cliché Storm: Main character has a friend who's a glutton? Check. His other friend is a brainiac? Check. The three main protagonists get bullied by some other characters and feel like they want to do something big? Check. An elderly relative who's always had a dream but could never accomplish it? Check. A person from the elderly person's memories returns after years to help the protagonists? Check.
Fridge Horror: At the very beginning of the film, we see a normal, realistic dragonfly during the prologue, which shows a monkey being sent into space. The insect characters in the rest of the film, however, are anthropomorphic. Did the radiation caused by the rocket somehow mutate them?
So Okay, It's Average: The film was quickly forgotten about, passed off as just another straight to DVD kid's film. It currently has a 4.6 on IDMB, 19% on Rotten Tomatoes, and a 36% on Metacritic, indicating nobody seems to care for it.
Unintentional Uncanny Valley: Or maybe not so much. Even for it's time the CGI has not aged all that well with some finding the flies rather erm, creepy.
. Who knew that Akari Kitou could sing that fast?