Jean Grey. The moment they reversed that, the door was open. By the time of Superman, everyone who was a regular reader of comics knew he wasn't dead or not for very long. Only the clueless media and speculators thought it was final.

Death was never that permanent in comics. Too many villains had come back from seeming final endings. The Comet, one of the first heroes to be killed, was back, in the Silver Age, though few noticed. Bucky was one of the few that stuck for a long period of time; but, there were teases, with people like Rick Jones taking up the role.

There were few deaths in comics and mostly old characters who weren't being used. DC killed the Batman and Catwoman of Earth-2, to give the Huntress more of a backstory; but, they weren't in use because the Earth-1 characters were dominant. They killed Mr Terrific when no one cared and most had no clue about the character. He wasn't much of a character, to begin with, until James Robinson got his hands on him and showed how you could make a guy who succeeds at everything interesting. With Jean Grey, it was a big deal. There was a body, there was a death scene, there was mourning. We had teases of her return; but, they were revealed as hoaxes. It was final that she was dead. Then, she wasn't. It wasn't a spectacular storyline that brought her back, either. She was revived to launch X-Factor, a rather mundane series, despite Walt Simonson's involvement. It would have been a bigger deal had Madeline Pryor been revealed to actually be Jean reincarnated. No, Jean's so-very definite death and quick stunt return is where I feel that death, as a story device, went out the window.

No, Wein's Rule was a constant: No one is dead unless you see a body and not always then.

Really, the only deaths that have held are civilians that are required for the her origin: the Wayne's, Jor-El and Lara, and Uncle Ben. After that, the only hero death of consequence, that has stuck, is Mar-Vell. Even when Thanos, Pip, Gamora, and Warlock & Drax were all revived, an issue of Silver Surfer affirmed that Mar-Vell was well and truly dead, leaving a continued line of lesser Captain Marvel's, related and unrelated.

Sadly ironic, too, that Chris Claremont did not want her to die AND did not want her brought back. But who cared what the guy who made The X-Men The X-Men wanted, right?

In terms of willing suspension of disbelief, it was a disaster. Nobody cares who dies in comics nowadays, because any character will be brought back the second enough readers ask for it or a writer wants to use them.

Dramatically, it also deprives readers of a very poignant part of life, even if it is only the life of make-believe characters: that of mourning.

The death of Jean Grey and the way her boyfriend picked up the pieces of his life was a powerful and ultimately hopeful story.

The death of Guardian in Alpha Flight was cut from the same cloth, and may have been even more powerful because he was basically the lynchpin of the series. What do you do when the focal point of your life vanishes? The way his wife and friends carried on was moving, and was a great lesson to young readers who may not have known major losses yet.

Nowadays there is no mourning. Characters may gnash their teeth, but it is a staged emotion, nothing more. As Agent Brand said when apprised of Jean Grey’s latest death, “yeah, that will last”. We have replaced melancholia with cynicism.

Marvel did this with many of their major characters in the late 1980's. Tony Stark was replaced by Rhodey, Steve Rogers was replaced by Johnny Walker, and Thor was replaced with Eric Masterson, one right after the other. Dc's run of doing this with Superman, Batman, and Wonder Woman, was a couple years later.

The first that comes to mind is Captain America becoming Nomad in Captain America #180 back in the early 1970's, with a series of replacement figures trying and failing to become the new Cap. That was a short, one-off storyline, as he was back in the costume by #184, but the themes in it - the person inside the costume is more important than the costume - remained the same for all the longer, later versions. 

I'd say Hal, but would you even really able to classify his as a "death"? All he really did was become a newer iteration of the Spectre once Parallax got free

Jean Grey’s death, in the long term, and more importantly her return, did make death in comics irrelevant.

But in the short term, her demise also ushered in a deplorable trend: that of killing important characters for shock value.

Karate Kid, almost the entire supporting cast of Rom, Glenn Talbot, Thunderbolt Ross, Spider-Woman, Biotron, Ned Leeds... and I’m not even counting secondary characters like everyone Scourge murdered. Heck, even whole countries, planets or universes would be sacrificed... Bye-bye, Microverse; adieu, Counter-Earth; thanks for the memories, Savage Land. (And multiple Earths? What are they?)

Such deaths and cataclysms made the c9miccomic-book universes poorer, not richer.