Alternate Character Interpretation: Not so much the characters as the writers. Gerry Conway has been accused by many as killing her off because he preferred Mary Jane, but the official story is that they wanted a shocking death, and Gwen was the only one who would be shocking enough. So for many fans, it's down to two interpretations: a case of Die for Our Ship, or a case of Stuffed into the Fridge. Gerry Conway himself is unambiguous:
"The amazing thing was that he [Stan Lee] created a character like Mary Jane Watson, who was probably the most interesting female character in comics, and he never used her to the extent that he could have. Instead of Peter Parkerís girlfriend, he made her Peter Parkerís best friendís girlfriend. Which is so wrong, and so stupid, and such a waste. So killing Gwen was a totally logical if not inevitable choice."
Spidey's No-Holds-Barred Beatdown of the Green Goblin after Gwen dies. Keep in mind, Spidey is sick during this fight (only a cold, but still), and yet he manages to wipe the floor with the Goblin. He was that pissed!
On a meta level, the fact that the writers managed to permanently kill a major character convincingly. Though as pointed out by Gerry Conway, the decision to kill Gwen was not especially controversial by the editorial staff.
Likewise, by having Peter fail to save his Love Interest, the writers set up such a Shocking Defeat Legacy for Peter that they couldn't add on to it without making him too much of a Failure Hero. Conway's idea of killing Gwen was merely a one-time solution to the problem of whether or not Peter should age. Later writers tried to mimic it, with the death of at least one would-be love-interest ("The Death of Jean DeWolff") whose connection to Peter and circumstances of her actual death added to Survivor's Guilt for Peter but not the same sense of failure. Likewise, after Peter and MJ got married, they tried to spin a story that Mary Jane had actually been killed in an aircraft accident, albeit in a Never Found the Body manner that added to Peter's frustration and his failure, making him such a sad-sack that it made his books cross into Darkness-Induced Audience Apathy.
In general, the Gwen Stacy story is more or less part of the long-term of issue of whether Spider-Man should grow up past Comic-Book Time or remain young and hip and relatable. Later writers in grappling with the problem and also trying to keep Spider-Man single unleashed the true Dork Age of Spider-Man leading to controversial retcons like the one in The Clone Saga and universally reviled storylines like One More Day, both being extreme ideas to keep Spider-Man from aging and freeing him from being part of an Official Couple.
The storyline also becomes a problem when it gets adapted, because the main reason it happened was that the writer and Marvel's editorial team considered Gwen too bland and uninteresting, and so expendable, yet on account of being the Designated Love Interest, she was treated after her death as Peter's The Lost Lenore which made later writers try to rework and adapt Gwen into a more complex character than she actually was, with many of their revisions amounting to In-Name-Only takes, treating Gwen's death as if it was like killing Lois Lanenote Remember Superman turning back time after Lois dies in Superman, that's how much audiences and creators expect their protagonists to cope when they lose someone they deeply and truly care about on a profound level and why most would prefer to avoid such situations if they can help it. As anyone who follows serial narrative know, essential supporting characters only face Character Death when nearing a conclusion (such as a "last superhero story"note (as in Batman: The Dark Knight Returns, Whatever Happened to the Man of Tomorrow?, Logan or Spider-Man: Reign which is the one of the few AU stories where Mary-Jane dies, featuring an old Peter Parker), but Gwen's death in the original stories was more or less a quick sudden event, and within a few issues Spider-Man was back to his old quippy self. As such when in The Amazing Spider-Man Series they made Peter and Gwen's romance into an epic love story and transform Gwen's personality entirelynote In that she knows Peter is Spider-Man, loves and accepts his double life, doesn't hold Captain George Stacy's death against him, and is warm, funny, and snarky like OG!Mary-Jane, they more or less had Peter quit being Spider-Man for a year, and then somehow hop back into action as the old quipster which many people saw as unbelievable for the story of a man who lost the love of his life, and become a complete and total failure as a hero, a fact which becomes apparent when one notes that the abandoned plans for the third film had Peter more or less becoming a Mad Scientist in trying to bring her back. Notably, Greg Weissmann, the creator of The Spectacular Spider-Man (which also transformed Gwen and likewise made her into a likable supporting character) stated that he had no plans for killing off his version of Gwen, because she had become a much more complex and far less disposable character. Likewise, Ultimate Spider-Man did a take on Gwen Stacy that initially saw them killing off Gwen but later bringing her back as a clone-but-not-really-and-as-good as the real Gwen and treated her as Gwen returned.
Harsher in Hindsight: When at the end of ASM #122 Mary Jane attempts to console the grief-stricken Peter and mentions how she herself is wrought up by Gwen's death, he tries to brush her off by saying she would not even care if her own mother died. About ten years later she would reveal that her mother had already died before Peter first laid eyes on Mary Jane.
Hype Backlash: On account of its status as "the best Spider-Man" story and so on. Recent fans have deprecated the story, seeing it less as a daring Reality Ensues moment, than the first of Marvel's Writer on Board attempts to keep Peter from maturing and growing up. They also note that read in context, the plot comes across as sped up and artificialnote Norman Osborn after being dormant in amnesia just suddenly snaps and the plot itself has no build-up and tension and just proceeds from there and the art-work aside from some famous panels is not as great as Spider-Man at his best. Especially with the popularity of Spider-Gwen and the popularity of Emma Stone's performance in The Amazing Spider-Man Seriesnote To the point that her death in the film was poorly recieved and the film itself was highly unpopular there have been calls to resurrect Gwen Stacy for good.
It Was His Sled: This trope is clearly Up to Eleven considering the spoiler is in the title itself. However, the title itself isn't shown until the end of the issue in which she dies... But good luck trying to read this story without hearing about the title from a trade paperback it's sold in.
Like You Would Really Do It: The writers made it absolutely clear that Gwen Stacy was dead for good. The same applied to Norman Osborn until he was resurrected over twenty years later. Dampening the effect is that Gwen had become a Designated Love Interest at that point, and the Green Goblin had made more appearances as the amnesiac Norman Osborn than as a villain for the entire John Romita Sr. era and was a rarely used villain at that point. What made it shocking was that the Spider-Man comics in general, and superhero comics on the whole, were usually not so violent at the time.
Moral Event Horizon: The Goblin's murder of Gwen is perhaps the best known example for a Moral Event Horizon in the history of comic books. Not only because of the act itself, but because of his utter dismissal and even contempt for the value of Gwen's life.
Some fans define the character of Norman Osborn by the crime he committed in this story, and less-informed fans define Gwen Stacy by this moment/her characterization during this moment. She's almost always remembered as the Girl Next Door type characterization that she had at the time, and remembered as the 'one who died'. At least in Gwen's case, Spider-Gwen has helped define her character differently to a new generation of readers.
For a long time, this became one for Peter, cementing his Failure Hero status. In Civil War, Iron Man even refers to this instance as something that could have been avoided had Peter been registered and given training when he was young and inexperienced, while Alex Ross' Marvels elevates Spider-Man's failure to save Gwen as the ultimate End of an Age.
The story's reception at the time it was made was divided. The fans flooded Marvel with letters, some praising the creators for taking such a bold move, others calling them murderers. It was regarded later as a bold step for a company to kill-off a prominent supporting character and make the hero genuinely fail. Alex Ross saw it as the end of the Bronze Age of Comics and made it the climax of Marvels.
Since The Oughties-onwards however, the story has been regarded less kindly for codifying Stuffed into the Fridge (although Seinfeld Is Unfunny is in play here), and in the wake of the popular revisions of Bucky Barnes and likewise, Jason Todd, the idea of All Deaths Final within comic books is no longer considered a special achievement, and likewise when the story was finally adapted for the first time in any media (The Amazing Spider-Man 2), it was heavily disliked, be it mostly because the at least somewhat understandable reasons for killing Gwen in the comics (i.e. she was bland, somewhat one-dimensional, and made Peter feel guilty about being Spider-Man) simply didn't existnote Because the Gwen of the movies knew Peter was Spider-Man, accepted his double-life and was popular with the public (being the most beloved character of the Marc Webb movies. The popularity of Spider-Gwen has also diminished the appeal of the story, since it proves the character can exist without being either Love Interest or Stuffed into the Fridge, and likewise the shoehorning of ridiculous retcons in JMS' Spider-Man and Dan Slott's Spider-Man simply to either milk mileage from the story or pile on misery and guilt on Peter, just makes readers wish that Marvel simply bring her Back from the Dead already.
At the time the story was released, The Hero failing to save the Love Interest and said love interest getting killed for shock value was considered an unprecedented and bold move, especially since the Amazing Spider-Man comics at the time were known mainly for its low-stakes stories where there wasn't so much violence and death. It promised introducing real stakes and consequences and giving a sense that Anyone Can Die, and this was a good decade before Frank Miller's Daredevil, The Killing Joke and A Death in the Family.
Peter dealing with the grief and moving on with his life is considered a hallmark for his Character Development to this day, as well as Mary Jane Watson and the other supporting cast, who were changed by this single event. Since then, killing love interests for the sake of giving a protagonist a hard time has become so cliché that an entire trope is dedicated to it, and this story can come across as somewhat bland and even insulting for modern readers, as mentioned under Popularity Polynomial and Hype Backlash.