There's the argument that Amazing Spider-Man #121 transformed Gwen Stacy into something she had never been before. Perhaps if she hadn't died, she just would have eventually become one of Peter's ex-girlfriends, like Betty Brant. Instead, she became a symbol of innocence and of slight controversy, with most questions devoted to one storyline, and the wisdom of reversing it.
I'm wondering if One More Day did the same thing for Mary Jane. An obvious difference is that The Night Gwen Stacy Died is widely considered one of the best Spider-Man stories, while One More Day's flaws are acknowledged by many of those who think retconning the marriage was a good idea. This is also a situation in which the readers remember something the characters don't.
But looking at the Mary Jane Appreciation thread, a lot of what people like about the character is based on either an idealized version, or what could have been. It's the legend of Mary Jane, rather than what was on the page.
For example, this illustration by lone_wolf is beautifully done, but doesn't reflect anything that happened in the regular comics.
There was also a gag based on the idea that Gwen and MJ were in love with Peter in high school, when they never went to high school together. That sort of stuff.