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The NerveGear has medicinal value for the terminally ill—it can slow the progress of some diseases by conserving energy, and they can experience things in the game that their illness prevents them from doing in real life. This is actually a key plot point in Book 7 of the light novels, as Yuuki and the other Sleeping Knights are all hospitalized for various reasons—Yuuki is terminally ill with AIDS, for instance.
According to the novels, Asuna never had an interest in food in real life. So why did she become a master chef? Possibly because of the simple gesture of Kirito giving her cream for her bread.
Not only that, but she's grown up having food prepared for her; in Aincrad, she's preparing her own food. And as everyone who has ever cooked knows, there's a differences in satisfaction in eating something someone has made for you, and something you've made with your own hands.
There is a brief mention in the first volume about cooking in Aincrad not being as fun as real life, suggesting she might have cooked some in the real world. Considering the fact she does so at the end of the Fairy Dance arc this isn't impossible. This means there is another reason behind her mastering that skill; it was something familiar to her.
Alternatively she took up the skill because she wanted to find something she could relate to from the real world. Going off of the above, then naturally cooking would work for her.
As a genre-savvy young girl, Asuna also knows the best way to get her guy is Through His Stomach, and Asuna brings up her newfound skills wherever Kirito is around, offering to cook his rare Ragout Rabbit and insisting to share her food with him wherever they're in mission. Since Kirito had shown her some kindness, she was trying her hardest to impress him.
The whole incident with the 75th Floor Boss is a deliberate echo of the Black Cats of the Full Moon, with Asuna representing all of Kirito's comrades. Which is why Kirito loses his fighting spirit when he sees Asuna struck down. He loves her more than anything in either world, and the thing he most wanted to make sure never happened again happened again right in front of him, and again he was unable to do anything about it.
Why is it that SAO lacks a traditional Magic system, unlike other RP Gs? Because Kayaba wanted to keep this game as relatively close to real life as possible! It'd be hard to forget your in a game when people are hurling lightning bolts at each other!
Since SAO lacked any sort of government, Kirito decided to become Sheriff. That's why he's called upon to investigate murders. On that note, Sword Art Online invokes a Western trope of the sheriff confronting the corrupt mayor. Not just western: basically Kirito acts like the lone samurai of your average jidai-geki: cue to Mito Komon, where the dismissive, non-confrontational nice guy just tags along to witness something evil and then, after a Badass Boast he opens a can of whoop-ass on the bad guys.
Yui looks quite a bit like what you'd expect a biological daughter of Kirito and Asuna to look like: She has Asuna's features, but Kirito's hair and skin tone.
It's shown that Kirito has more sympathy for Kayaba than for Sugou, even though Kayaba harmed more people than Sugou did. The reason behind this is because Kayaba truly loved SAO and the VMMORPG genre and, while he indeed killed more than 4000 people, it was a restricted case. Sugou, on the other hand, only saw ALO as a tool for his ambitions and vainglory.
The second arc, particularly in the anime due to it being a more visual medium, has a huge spike in fanservice, with Gainaxing and Chainmail Bikini armour everywhere. In ALO, everything is run by Sugou, the creepy misogynist who sniffs comatose girls' hair. The fanservice spike is because he designed the game that way.
Leafa's reaction to Recon's Love Confession probably had something to do with the fact that he sounded like he was asking her to marry him out of the blue.
Much flak is given to Asuna for being a Distressed Damsel, and not even trying to escape. But consider her situation: Stuck in ALO by Sugou, who has full GM privileges, and he is not one to disregard advantages. So she's forced into a revealing costume, placed into a cage that she cannot break/affect ('immortal' object by all rights), and literally has a blur over her vision whenever the exit code is inputted, though not when viewed through a mirror; that we know for certain. Considering Sugou's personality, he has also most likely placed a barrier around the cage, to prevent her from slipping through the bars - or alternatively, even if she did, falling down and dying might just respawn her back in the cage. And would even he allow her to keep her ALO skills and abilities to ambush him? Simply put, there really was little she could do, and she clearly hated this... but she held out anyway, and nearly got out despite all this.
On the first day of SAO, Kirito refused to escort all of Klein's friends, for fear that he wouldn't be able to protect all of them, and figuring that he would blame himself if any of them died. Thing is, this is a flashback being narrated by Kirito after the 74th floor has been opened. Is Kirito recalling his actual feelings from that day, or is he projecting his memories of the Black Cats of the Full Moon onto his memories of that day?
In the "Alfheim" arc, Kayaba returning Back from the Dead to grant Kirito his admin account and powers and the World Seed may seem a cheap way to give closure to the plot. But actually, from Kayaba's perspective was the best thing to do. Despite being clearly victimized by Kayaba, Kirito still tried his best to enjoy and protect Aincrad, his finest creation. On the other hand Sugou, motivated only by greed, his delusions of adequacy towards Kayaba and his vilest instincts acquired Kayaba's patents just to squander them into a pale, vile imitation of his dream and weaponize his research. Basically, Kayaba used Kirito as his physical proxy and champion to wrest his uncorrupted dream from the hands of a thief. Kirito in the anime pretty much sums that up, claiming in his Badass Boast that, while both him and Sugou suffered at Kayaba's hands, Kirito was forged into the Hero of Aincrad and Sugou became a petty, green-eyed crook.
During the opening credits for the "Gun Gale Online" arc, Kirito can be seen writhing behind Death Gun while surrounded being circled by three shadows. This could be foreshadowing that Death Gun is actually three people.
It's could also represent the three Laughing Coffin members that he killed.
Something that may be obvious to some, but others might've not realized it, when Heathcliff kills Asuna, her sword is left on the ground and Kirito picks it up to use, people might wonder why it didn't disappear since she was dead, but Kirito and Asuna were married and shared an inventory, so Lambent light would've instantly become Kirito's the moment she died.
Why does Kirito hurry to Sinon's apartment even though she told him not to? Because of the Black Cats of the Full Moon. He learned from that incident to always error on the side of caution. No sense taking the chance that the murderer might still be there or set her apartment on fire or something like that.
Everyone referring to Freyja as a trap during Calibur. She is, in the slang sense.
The only girls in the crew that Yui tolerates besides Asuna are Leafa and Sinon. Why? Because Leafa is family and Yui can sense that Sinon doesn't have any romantic feelings for Kirito, but she can sense that the others do.
Kirito fought a duel against Asuna to dissuade her from going through with her plan to lure out the boss into a village and defeat it while it's busy killing NPCs, because that would be cruel. The Fridge Horror is that based on everything we know about bosses and NPCs in Sword Art Online, that plan had no chance of working. If they had gone through with that plan, it is likely that it would have gone horribly wrong. If Kirito hadn't been so protective of the NPCs, a bunch of raid members might have died attempting an impossible strategy.
After the arrest of two of the Death Gun conspirators, Kikuoka says that he expects them to get off on an insanity defense. Thing is, what they were demonstrating was not criminal insanity, but rather classic Serial Killer behavior, having complete control of their faculties right up until their plans fell apart AFTER killing four people. This means that a couple of serial killers are probably going to be released in a few years after being "cured" because they were never insane in the first place. It seems the Japanese justice system needs to buy a clue: trapping 10,000 people in your death game so you can play God without any appreciation for the lives of the people you trapped is criminally insane; making a game out of killing people when you know exactly what you're doing and you're just angry at the world is not insane, but rather typical serial killer behavior.
In retrospect, it's probably a good thing that Kayaba's ex-girlfriend decided not to kill him when she had the chance. See, there's no guarantee that killing Kayaba wouldn't have made SAO unwinnable, since that would have killed the final boss without defeating him in battle. If she had killed him, there's a distinct possibility it would have trapped all the players in the game forever.
Kyouji probably would have gotten away with it if he hadn't had that creepy look on his face. He had an alibi when Death Gun appeared in the BOB preliminaries, as he was logged into GGO as Spiegal, and both Kirito and Shino had seen him there. The only potential witness against him would be his brother, who would not be credible in the face of witnesses giving him an alibi. When Kirito and/or the police showed up at Shino's apartment, Kyouji could claim to have fought off the Death Gun accomplice himself and pin everything on Johnny Black. Had he not had that creepy look on his face, it's likely that Shino would have accepted his proposal, and it's not like he was going to rape her on account of not agreeing to have sex with him then and there.
What was going on in the Shinkawa family that they managed to raise two serial killers? You'd think a doctor with his own hospital would have noticed something and intervened before the Death Gun incident.