Excellent points raised by all. This topic is worthy of research. I'd love for a list of deaths/resurrections to be graphed so we could see a pattern to see which rebirth really started it, and the frequency by year. I have some memorable rebirths in my mind, but I need more info.

At what point were the floodgates opened where deaths became a gimmick? At what point was I no longer tricked by Marvel and DC when it came to believing "permanent death"? What year did we "wise up"? The OP is making the case that The Death of Superman killed comic books, and not the Image guys. But IMHO that's a different topic.

The consensus here appears to be Jean Grey's retconned "resurrection" (1986) but I see the point raised in this thread questioning if just being the first hyped resurrection made death in comics irrelevant.

I believe we had been conditioned subconsciously over the years before 1986 to not accept permadeath due to super-villains "dying" when they were defeated but always come back next time. So switching the trope to super-heroes- looking back- should not have been that much of a stretch (but it was for some reason). I also believe Nick Fury and S.H.I.E.L.D. used LMDs way too much, even before 1986. (And, as someone mentioned, it was not cool when Professor X returned, especially since it set his own trope of "dying" and returning). Gwen Stacy's clones, starting in 1975: not cool. IMHO all of those stories and similar ones with time traveling, alternate earths, robots, behind-the-scenes scheming, etc. subconsciously prepared us for what we have now.

DC used to keep the company line and ignore pre-Crisis material to the best of its ability (Grant Morrison's Animal Run being a notable exception) before Zero Hour 1994. Yet we still had IP creep in, like Byrne's 1987 Superboy/Legion/Time Trapper storyline and the new Supergirl (Matrix) in 1988.

The Beyonder should have stayed dead in Secret Wars II #9 (1986)- he had become a mortal baby and died. Jim Shooter had left and there was no need to revisit his pet character. Yet he returned in 1988. The Original Human Torch became reactivated in 1989. (How many times has the Vision or Ultron "died", too?)

Superman's 1993 death is the one which reeks of major calculated manipulation because of the mainstream media, speculators, sales, and how the storyline was milked for as long as possible. It was so financially successful that it inspired the "Breaking of the Bat" with Knightfall (1993-1994) and Emerald Twilight (1994), where Hal Jordan went psycho and was replaced. Mark Waid's 1993 Return of Barry Allen was a good teaser, but also proof back then that bringing Barry Allen back (2008) ought to have been unthinkable considering Wally West had earned the mantle.

Marvel got into the act too, pretending that Mr. Fantastic and Dr. Doom died in 1994. 

In 1995, Peter David penned Nick Fury's funeral in Incredible Hulk. It was an in-universe parody of how the heroes feel about going to another one of these funerals, knowing that he's probably a LMD.

Bringing back Magneto (too many times to cite) and especially Colossus (2004- years after he saved mutants from the Legacy Virus) was insulting. Any time a hero sacrifices himself or herself but is brought back, it takes away from the original story (see Supergirl and Flash in Crisis #7 and #8).

All this said, Jason Todd/Red Hood and Bucky Barnes/Winter Soldier- both in 2005 was the straw that broke the camel's back for me. Never again could I trust Marvel and DC. For DC, they ran a 900 number and 10,000+ fans paid to kill off Jason Todd in 1988. No refund given, of course. For Marvel, as someone posted, Bucky and Uncle Ben were the two untouchables. I had read this on letter pages before, and it was in the comic book zeitgeist. Although the storyline went over well, and Marvel made tons of cash off Winter Soldier and Cap since then, it should not detract from the audacity that it took to bring him back. Bucky and Jason were young civilian sidekicks with no powers. We all accepted their deaths as permanent and that was fine.

So to answer the OP, which rebirth made death meaningless? I'll probably side with the majority and go with Jean Grey because of the emotion of the original story, and her relationship with Cyclops. However, I have a nagging suspicion it was all the times we've read Professor X either dying, being paralyzed, mortally wounded, or "gone", not to mention almost every super-villain in comicdom, Nick Fury's LMDs, and the nature of DC and Marvel's multiverse in general (Crisis + What If? = multiple versions running around and dying).

When did everyone copy the gimmick? After the success of The Death of Superman.

When did I wise up? I am not ashamed to admit that 2005 was it for me, with Jason Todd and Bucky Barnes.