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: A Certain Magical Index
Alternate Character Interpretation: Accelerator: A young man who's trying to redeem himself for the terrible things he has done while trying to avoid turning back to his past self or a sadistic heartless monster who hasn't changed his ways and enjoys being a powerful killer.
Base Breaker: There are two separate camps for viewing Accelerator. One, the people who focus on his lethal methods who makes of him a bloody monster. Two, the people who focus on him trying hard to redeem himself even if that means throwing away his life.
Critical Research Failure: The reason why Stiyl looks so old despite supposedly only 14 was because of a miscommunication between the artist and author: Touma' and Stiyl's first confrontation was supposed be something of an Old vs New (which the artist took literally to mean "old man vs young boy"), completely missing the note that said Stiyl was only 14.
Designated Hero: As Aiwass commented Shiage's more or less a powerless nobody, but when it comes to those he loves Aleister's plans, or even pre-destination for that matter, doesn't haveshiton him.
Draco in Leather Pants: In-universe example with Accelerator as Last Order believes that he's a good person deep down, despite killing thousands of the Misaka clones. She theorizes that the taunts he made at the clones were designed to scare them and avoid conflict.
Now with the Second Season of Index, Fukiyose Seiri has become very popular with fans on her debut in Episode 8.
Fanon: Fans have speculated that the Amakusa Catholic girl that Touma fought and saved from a fall in Volume 7 may be Itsuwa, the Amakusa warrior who was first introduced in Volume 11 which may explain why she has a crush on Touma in the first place. However, the 2nd season anime had it Jossed as it wasn't Itsuwa that Touma saved but another girl.
Foe Yay: Touma and pretty much all of his female opponents. And some of his male ones. Accelerator and Fiamma in particular.
"Funny Aneurysm" Moment: Index-tan Episode 2: Himegami is asked what her wish for the second season is. Her response? "Um, I won't ask for my episode, but I just want to be treated more-". Guess what happens in her first lengthy appearance in season 2? She gets filleted by Oriana Thomson because she thought that Himegami was an agent of Necessarius due to the Celtic cross around her neck.
Harsher in Hindsight: Academy City's Power Curriculum Program. In theory anyone undergoing this program can become an esper yet there are still people who have yet to gain powers and even those who did never go a higher level which suggest the program has some problems. However Vol. 22 reveals the program is secretly being undermined by the Higher Ups of Academy City who decide who can and cannot gain esper powers and/or go a higher level based on the type of the powers that person has. This revelation makes it much worse once you realize Level 0's like Saten, the Skill Outs gangs and low level espers like the Level Upper users, who work hard to gain powers/higher levels but never can because their training is being sabotaged as the Higher Ups see no point of helping anyone who doesn't have the powers they want. That's right, the city is responsible of destroying the dreams of many students and leading them into path of cynicism and crime!
All intentional; while Mikoto claims in SS: Liberal Arts City that Academy City has no intentions of taking the world via military force, the revelation via Chapter 22 suggests otherwise. Why else would you try to make an army of level 5s or an "interceptor" aircraft that has the ability to fly halfway around the world and level a city single-handedly? Basically, the reason why the corporate crooks of AC haven't bothered leveling the level zeros of their population isn't just because they don't want to, its because they need cannon fodder; they needed a group of disgruntled individuals who they knew would resort to violence in order to defend themselves against those termed and rewarded for being genetically superior, so that they might label that group as people on whom it is okay to use your powers, people whom it is okay to murder, because no one will care about them due to their collective reputation.
Ho Yay / Foe Yay: Fangirls love shipping Kakine with Accelerator.
Not to mention Touma and Accelerator have some Foe Yay and Ho Yay as well.
Touma and Fiamma of the Right.
Shiage with either Touma, or Accelerator. But more on the Foe Yay side.
Iron Woobie: Touma gets his ass kicked on a regular basis, and even the fights he wins tend to end with him in the hospital. There's no real explanation for why he puts himself through such misery, especially considering he has his own inherent bad luck to deal with. And yet he claims to be happy with way he lived his life...
But he always does win in the end, and women fall like flies to his two-dimensional Shonen Protagonist hero speeches. What's not to be happy about?
Narm: Apparently, Accelerator can state-change air into plasma by making ridiculous noises with his mouth. To be fair, the ridiculous noises are another form of his evil laugh.
This is also a case of Formulaic Magic, those noises he was making were Accelerator performing the calculations necessary to use his Vector powers at high speeds.
Never Live It Down: No matter how much good he does or how many heroic acts he does, some fans still hate and can never forgive Accelerator for the terrible things that he's done especially what he did to the Sister Clones before his Heel Face Turn.
Ships That Pass In The Night- Surprisingly, fans like to pair Touma from Index with Saten from Railgun because they're both normal, even though both characters have never met each other in their respective series.
At least until chapter 58 of the manga.
There's also a minor one with Touma and Mitsuko.
And a very small Touma and Kuroko one, though this is more of a Crack Pairing.
To be fair with the Touma/Kuroko shipping, they did have a short moment during the Remnant arc of the second season. It was here that most fanart between the two started cropping up.
Sure if you're talking about fanarts, but in terms of fanfics... this troper has seen as much as Touma/Kuroko as how much Touma/Mikoto totals up to. In 2ch. And a lot of them would like Kuroko turning straight. Still a Crack Pairing yes, but with the events based off on Volume 8 now animated, that doesn't help to fortify that fact much further anymore.
It's rather surprising how many fans are starting to pair up Fiamma and Carissa, considering their single canon interaction was an I Shall Taunt You on Fiamma's part.
Ship-to-Ship Combat: Which girl should be with Touma? Don't ask! because the moment you say so, you will get yourself in a nasty Ship-to-Ship Combat between other fans over their favorite girl!
Yet two of the biggest Ship-to-Ship Combat by Japaneses fans are the Mikoto x Touma fans and Kaori x Touma fans. These two fanbases are so big, the novel artist Kiyotaka Haimura knows it and even drew about it!