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.
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: Averted - 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.
Love It or Hate It: The story's reception at the time it was made. The fans flooded Marvel with letters, some praising the creators for taking such a bold move, others calling them murderers. Nowadays, it's highly regarded.
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.
Tear Jerker: Gwen's death and everyone's reaction to it.
Spider-Man: "I saved you, honey... don't you see? *quietly* I saved you..."