The means by which Jean returned to the dead have become particularly notorious over the years for being "too complicated." I'm not sure they are, not compared to some of the ridiculous ways characters have been resurrected. Heck, in 1970 X-Men fans accepted the explanation of how Professor X came back from the dead and it was nuts. Following Jean, Iron Fist went through an extremely complex resurrection but no one gives grief to Iron Fist over it. No, comic book fandom holds the resurrection of Jean Grey with a little contempt and that's really what they mean when they call her return "too complicated." Xavier & Iron Fist's deaths simply weren't considered great stories - unlike the Dark Phoenix Saga. Sure, comic book characters died and came back from time to time - but never before had it happened to a story which was so greatly admired.
Still, the demands of nostalgia brought Jean back - back to a world which had changed in her absence. Although Bob Layton's X-Factor was hep to get the original five X-Men back together and ignore Uncanny X-Men & New Mutants for as long as humanly possible, and although Beast quickly returned to his 60s personality, while Angel found himself attracted to Jean again (a dropped bit from the 60s), time had moved on. Cyclops was cut off from Jean (for reasons I'll dig into tomorrow). Jean heard from Reed Richards that in her absence, the X-Men had begun working with her old enemy Magneto (Reed evidently never thought to mention he'd also worked with Magneto during Secret Wars, where the X-Men/Magneto alliance originated; nor did Jean think to ask Storm as a friend to explain the alliance to her - or even tell Storm she'd returned!) and anti-mutant hysteria seemed much stronger than before. Jean's determination to make the world a better place for mutants is what ignited the concept of X-Factor.
There's something about super hero resurrections which inevitably invites comparison to Christianity. As in the death of Jesus, when a popular hero is dead characters enter into that twilight of the soul and angst, "With Captain America/Iron Man/Reed Richards/Gilgamesh dead, how can we carry on in their absence?" Then a year later Captain America/Iron Man/Reed Richards/Gilgamesh's second coming arrives with the promise that the super heroes' world is going to get better. Even though - let's face it - that's really up to the whims of the writers and editors. If the only hero left on Marvel's Earth were Captain Ultra I'm sure everything would still work out.
But Jean - for all the work done to get her back into circulation - had returned to a Marvel Universe which didn't need her. Madelyne Pryor was Cyclops' new love interest and Cyke seemed much the better for it; Rachel Summers was in the midst of an ongoing plot which seemed to point towards an idea beyond the limits of the Dark Phoenix Saga, rather than retreading old ground; and Jean Grey herself, although ostensibly still going by the codename "Marvel Girl," seemed to have aged well beyond the years of "girlhood" ("Marvel Girl" would appear infrequently during the total run of X-Factor; when Jean then rejoined the X-Men it became clear there that she had no codename and, except for attempts to turn her into Phoenix, she hasn't had one since).
Jean's return excused bringing the original five X-Men back together, but Jean had little else to offer, especially in the Layton issues where she was strangely unconcerned about any of her old friends and family beyond the old team. She did bring a lot of friction between her and her old friends, but that potential would need Louise Simonson & Chris Claremont to explore it further. "People thought I was dead; now I'm back" isn't a hook which draws readers in - we're not likely to have been in that situation ourselves (barring any unfortunate clerical errors). At best, Jean's issue is "why did things change while I was gone?" Which, is decent enough baggage for a person to carry - enough to make me empathize, but not sympathize.
(I am now going to compliment Wonder Man; ready your fainting couch)
An advantage which Wonder Man's resurrection in 1976 holds over Jean's is that while both characters carried around trauma from their time spent in un-life and developed friction with people who had moved on during their absences, Wonder Man was for several years afraid of dying because of his experiences and that fear of death is something which is actually rather rational amidst the sort of life and death struggles super heroes are placed in. I sympathized with Wonder Man's baggage.
Jean Grey's resurrection and its "complications" predicted what would soon become a complaint directed at the entire X-Men franchise: "it's too complicated." By the end of the decade, Madelyne Pryor's true origin and Mr. Sinister would rewrite the book on exposition. The subsequent decade would see competing time travel stories with Cable, Bishop and the Askani; the Onslaught crossover would sour the very idea of super hero crossovers for years to come; Magneto would be replaced by Joseph; Colossus would turn evil, Marrow would turn good, Gambit would turn useless and somewhere in there is a garbled story about Sunspot being Reignfire.* The X-Men comic books may not have been any more complicated than any other super hero "family" of titles, pound for pound. However, the believed importance of Jean Grey's death and fandom's complicated reaction to her complicated return would give them a reputation - one which persists to this day.
*= also: Externals, Silver Fox, bone claws, Crimson Dawn, Tolliver, Apocalypse, Stryfe, Domino, Copycat, Maverick, Thieves Guild, Douglock, Phalanx, Upstarts, Blaquesmith, Age of Apocalypse, Monet St. Croix, Shatterstar...