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."
Complete Monster: The Green Goblin established himself as a completely monstrous individual with this story. As if killing poor Gwen wasn't enough, the Goblin completely mocks the value of her life. The point is driven home when the Goblin gets completely pissed after Spidey (supposedly) destroys his glider, and starts to cry even harder for revenge. Spidey calls him out for acting in such an entitled way when he just killed his girlfriend. The Goblin then says the little chestnut of calling Gwen "a simpering, pointless girl who never did more than occupy space". Cue Spider-Man beating the crap out of him.
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.
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.
Seinfeld Is Unfunny: This is the one comic that started the entire Woman In Refrigerators trend, predating the Trope Namer by a good two decades. The motivations for it behind-the-scenes were identical to the general trope, using the girl's death to add to the hero's Angst and provide him something to feel badly about.
At that time, where superhero comics were generally seen to lack consequences, a Wham Episode where the hero outright fails was quite new and unexpected and seen as a daring Reality Ensues. Unlike later examples such as Jason Todd or Sue Dibny, the story was promoted and presented as "just another Spider-Man adventure" and the finale felt like a Surprisingly Sudden Death to most readers. A one-time effect that is impossible to repeat or appreciate in retrospect especially after the dark age of comics.
Unlike other examples, Gwen's death had deep, meaningful consequences to Spider-Man's character and mythos, and Conway and later writers treated the death as Spider-Man's Shocking Defeat Legacy, inspiring him to be a better and more heroic character and elevating Gwen to respectfully become The Lost Lenore even as MJ became his Second Love. The trope it inspired is I Let Gwen Stacy Die and not "Dropped From A Bridge" for a reason.
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..."