And You Thought It Would Fail: Executives were certainly nervous about a costly movie starring a group of superheroes who had never been on the big screen before. And for frame of reference, this was a time when the failure of Batman & Robin was still fresh in everyone's minds, when Superman's fifth movie languished in Development Hell, and when the only successful Marvel movie had been 1998's Blade. Of course, there are stories (per Moriarty over at Ain't It Cool News) that executive Tom Rothman really opposed this project.
"Ive spoken to at least ten people close to the production who have provided me with laundry lists of the ways that Rothman tried to fuck up the first film. Remember when they cut the budget and moved up the release date on the first X-MEN? You know why? Rothman was cutting his losses. He really, truly anticipated that the film would come out and vanish without a trace, and he would finally be rid of what he saw as a corporate albatross. Instead, the film clicked, and on the second film, Bryan Singer and his writers and the producers were all able to muster enough muscle to get Fox to give them the room they needed to make something even better."
Genre Turning Point: The films opening scene with a brutal depiction of a concentration camp is now widely viewed as the moment people first saw that comic book movies could be more than just cheap action schlock.
Mystique sabotages Cerebro to put Xavier into a coma. Since X-Men: First Class is part of the same continuity, this means Mystique tried to kill a man whom she grew up with and loved as a brother and did not part with on hateful terms—and she did it all just to further an agenda.
Now that we know from X-Men: First Class how Erik's mother died and how Charles became a paraplegic, Magneto's contempt towards guns here makes perfect sense.
In X-Men: First Class, it initially seems odd that Charles wouldn't encourage Raven to pursue a formal education (she sardonically responds to Amy's "What do you study?" with "Waitressing"), but when you recall what Mystique had said to Senator Kelly ("People like you were the reason I was afraid to go to school as a child"), then it becomes very clear that she and Charles were extremely fearful about the possibility that she might lose control of her power while in class.
Professor X's line "Experimentation on mutants—it's not unheard of" becomes more chilling when you take into account Bolivar Trask's examination and dissection of mutants in X-Men: Days of Future Past. Xavier would be thinking more specifically of Banshee's death and the torture Mystique endured before she managed to escape Trask Industries in the original timeline.
Heartwarming in Hindsight: Professor X tells Logan, "I give you my word that I will use all my power to help you piece together what you've lost, and what you're looking for." Wolverine doesn't know it yet, but what he's truly looking for is right in front of him. As X-Men: Days of Future Past demonstrates, Xavier is the person who gives meaning to Logan's existence; the "master" to his "samurai," so to speak. This is especially true in the Alternate Timeline, where Wolverine has settled down as a history teacher at Charles' school, and is genuinely happy—probably the happiest we've seen him in the entire film franchise. Days of Future Past makes it clear that it's Xavier—and not Jean Grey, as The Wolverinemight have you believe—who is the most significant figure in Logan's life during his post-amnesia years.
Magneto playing with a bullet he's pushing into a policeman's forehead is very similar to how he killed Shaw with a coin back in 1962...
Magneto's comment in the aforementioned scene that he doesn't think he can stop all of the bullets becomes incredibly amusing given how many missiles he managed to catch in X-Men: First Class, suspended in mid-air, then returned back on their original trajectory.
Jean Grey tells Professor X that Logan's healing ability "makes his age impossible to determine. He could very well be older than you, Professor." She doesn't realize what a gigantic understatement this is because both X-Men Origins: Wolverine and X-Men: First Class have established that Wolverine is about a century older than Xavier! (James Howlett was born in the early 1830s while Charles' birth year is the early 1930s.)
Cyclops scoffing at the idea of mutants wearing yellow spandex is ironic, considering how in X-Men: Apocalypse Psylocke's outfit is made of spandex and serves no function outside of looking comic-book accurate.
Magneto is the film's main villain and is willing to sacrifice an innocent young woman in order to further his goal of mutating mankind, but his tragic backstory of being sent to a concentration camp during World War II makes it impossible not to feel at least some sympathy for him.
Senator Kelly is an anti mutant racist but he gets kidnapped by Magneto and turned into a mutant himself, which causes him a ton of pain throughout the movie. By the end of the movie it's hard not feel bad for him since even though a group of mutants helped him, another made his life a living hell until his horrific end.
Mystique kidnapping Senator Kelly is a serious scene, but her using her feet to threateningly grab his face causes it to lose some impact. Kelly looks like he is more disgusted at having a blue mutant's bare feet on his face, and Mystique looks more like a mistress dominating a slave with a foot fetish rather than an experienced agent kidnapping a politician.
Storm's Pre-Mortem One-Liner to Toad. Throughout the film, Toad was supposed to keep bragging about things a toad can do, but the lines were cut during production. As a result, Storm's quip lost all of its impact and instead sounded like a specially silly one-liner.
Relationship Writing Fumble: Logan and Rogue. They gave romantic vibes in their interactions, leading the viewer to think that they would end up like that. The film does at least lampshade it towards the end by making it look more like Rogue had a crush while Logan views it as a Like Brother and Sister relationship. By the sequel Bobby even has mild jealousy when Logan returns.
"Seinfeld" Is Unfunny: At the time of its release, both critics and mainstream audiences highly praised the film for bringing comic book movies back into a positive light. With its grounded and realistic look and gritty tone, it helped give the previously dead genre a jolt of life. Nowadays, many modern audiences criticize it for having clunky dialogue, a rushed pace, bland production design, and rather dull fight scenes in comparison to many of today's more polished and epic superhero movies. Still, the film at least deserves to be admired and respected for its bold take on a genre that was then considered to be box office and critical poison.
They Wasted a Perfectly Good Character: Sabretooth is fairly non-descript in this film, despite his animosity towards Wolverine being one of the most famous rivalries in comic book history. Sure, the two have a couple of fight scenes together, but the only significant interaction Creed has with any member of the X-Men is his creepy lust towards Storm.
They Wasted a Perfectly Good Plot: When Senator Kelly arrives at the school, he's in bad shape, leaking water all over the place and finally dying when his body rejects the false mutation, dissolving into water. But is that what happened? A line from Magneto, "Are you certain that you saw what you saw?", implies that on some level, the writers were thinking that Kelly just lost control over his powers and will eventually be able to pull himself back together. But, no, none of the future films followed up on this, and Senator Kelly really is dead.