Well, for one, Norman didn't die during the course of any ordinary story, he died in what is arguably one of the most significant comic book stories in history. And his death in that story was considered to be him paying the ultimate, and poetic, price for his most hienous sin. An eye for an eye, a tooth for a tooth, a life for a life... His death also set in motion other major events in the history of Spider-Man, such as Harry Osborn's eventual downfall and taking up the mantle of the Green Goblin himself, and the creation of the HobGoblin, who remains a favorite villain of many. Add to that the fact that his evil essence just seemed to hang over Peter Parker whenever a loved one was threatened as a result of Peter's life as Spider-Man, or whenever Harry freaked out, or whenever another pretender to the Goblin throne showed up. You could make the argument that Norman's villainy was more omnipresent while he was dead than when he was alive. Plus, the longer he stayed dead, the more "classic" the Death of Gwen Story became and the more "sacred" everything about that story became (including Gwen's increasing canonization as some sort of saint) that no one dared touch it until Bob Harras, who, either had the balls or the gall, depending upon your persepective, to bring Norman back. The argument became that he had been dead more than 20 years, and the Spider-Man comics went on and during some eras, thrived quite well, without him, so why was he needed now? Usually, we know right away that if a villain is killed off, he's coming back some day, because it's in the Supervillain Rule Book, but after 5, 10, 15 years, Norman's death was clearly not your normal supervillain death - it was part of a story that helped define the very hero he bedeviled - and a story that is often considered to represent the end of the Silver Age.
I can't speak for the circumstances surrounding the "deaths" of Doom and Magneto, but in the case of Doc Ock, for example, in neither case was his death "classic" or "poetic" by any means. His first "death" supposedly being blown up in a nuclear accident, occurred during a story with a really dumb idea, the whole "Doc Ock marrying Aunt May" thing. And his second, at the hands of Kaine, was stupid - it was solely to make Kaine look "cool," and that was admitted to by Glenn Greenburg - that Marvel wanted Kaine to look real badass, so it was like "let's have him kill Doc Ock. Coooooool." it was a disgraceful death for such a great villain, and once the hangover that was the Clone Saga was finally over, I think clearer minds admitted that it was a stupid death and wasted no time in bringing him back. In neither of those cases was his death the logical or natural outcome of the story - they were just plot devices
Plus, I think there's a lot deeper bench strength with Spidey's villains than maybe some of the other superheroes/teams. You don't always have to have Norman around, because you've got Doc Ock, Vulture, Electro, Venom, for awhile Harry was a bad guy - maybe all not in Norman's class, but the Ditko villains in particular are great villains and the writers can't resist using them. Norman could take a several year break and not be missed too badly. I don't know that you can say the same for the other heroes' rogues gallery, but I COULD BE WRONG. I think the movies are a perfect example of this. In the Spidey films, for example, while yes, Norman was present in all 3 films, he was the focus in only one. In the FF and X-Men films, you had the same villains in each one, as if the writers simply didn't know if they could make the films work without THE #1 bad guy of those teams. Kind of like the difference between the Batman and Superman films.
The whining about his return does get old, yes, there's some short-sightedness to it, and some of the arguments put forth are pretty flawed, but at least there is some logic to the argument.
Crap, if I say anymore - I should just write a damn essay!