Comic accurate presentation of the characters in terms of their individual personalities, use of powers and interactions but also in terms of the team ethos and the broader spirit of the X-men comics. X-men was NEVER about being super-heroes, it was about being human, being outsiders and eternally striving for the cause of hope and the greater good in spite of all the hate. That is what differentiated them from the Avengers and other groups.
As a contrast (not criticism!) in the MCU Scarlett Witch is said to be "a kid" but in reality she never once showed that on screen.
Nightcrawler in the comics is shy, retiring and there was nothing "super" about him when he first came on the scene. This was accurately and successfully demonstrated on screen (not via exposition) to a degree unmatched in any of the other X-films that NC has appeared in. Nightcrawler was a lost and scared spirit when he first encountered prof. X.
In the Comics, Jean and Storm are close, almost sister like, especially in the Byrne run. This was accurately portrayed in X2 and to a degree beyond any of the other X-movies. In the comics Jean Gray was turned by attacking her human side. Her humanity. The notionally "weakest" member of the HC turned the strongest X-man.
In the mansion attack the X-men are shown to be vulnerable humans and kids even (with godlike powers) which is comic accurate. Iceman, Pyro and Rogue needed Wolverine to "look after them".
The disdain and distrust that normal humans have for mutants was subtlety and effectively shown in the Iceman's family scene. Not through ham-fisted diatribe but instead through the old

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The closing epilogue and closing scenes were the closest X-men films have come to hinting at the power, the majesty and vulnerability of Phoenix. None of the other films with her come close. X3 and the squawking bird in Apocalypse were a joke in comparison.
Central to the comics was the "School for the gifted at Graymalkin Lane". It's a school. The X-men are learning not only to control their powers but how to be better people. "I thought Bobby was going to a school for the gifted" is a classic line 110% in line with the comics.
X-kids being kids and doing kids stuff was shown in the mall museum scene. Normal kids trying to bully them and them having to shown restraint (a theme in the comics) was accurately portrayed. The mall, going for Ice-cream, cinema, hanging out, watching TV etc etc were all running themes in the X-men where they just did normal human kid stuff. Example: Kitty Pride was having Ice-cream in a shop when the HC attacked.
etc.
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The X-men is more a human character drama or a humanity character drama than about powers. That is why IMO the FF will appear in the MCU before the X-men because the FF are a closer natural fit theme-wise.
First Class stans do not understand that the X-men is not about lights and powers. I should add that the portrayal of Shaw and the Hellfire Club alone should disqualify FC from ever being taken seriously. The Hellfire Club is about lineage, power, subtle influence and majesty, not anything as trite as super-powers and fighting wars.