The Phoenix Saga ruined the X-Men for a few years.
I know Jeff Lester disagrees with me on this, but he's wrong; as exciting and classic comics as it may be, the whole Dark Phoenix thing derailed UNCANNY X-MEN all the way through #175, and I'm blaming it all on Jim Shooter and John Byrne.
Okay, that's maybe not entirely fair - especially Byrne left the book within six months of the end of the storyline, and Shooter probably bears less responsibility than Claremont, who was, y'know, writing the book and all - but while everything from #125 through #137 has become Official Comic Landmark material because Claremont and Byrne are working in such sync and with such success that even introducing Dazzler can't slow them down, the following year is a pretty great example of watching a writer thrown entirely off his game.
That year between #125 and #137, though, is a great read; Claremont and Byrne are on fire, introducing the Hellfire Club, Emma Frost and Kitty Pryde as well as Dazzler, and keeping the main characters evolving (Colossus has to kill! Cyclops stands up to Professor Xavier because he knows the X-Men better!) even before the big cosmic showdown that sees a character turn, essentially, outright evil and then pay the price for it. The year seems like the fulfillment of basic Marvel ideals, mixing soap opera and superhero, showing the need for responsibility that comes with power and ending with a tragic self-sacrifice that "This Man, This Monster" would've been proud of. It's really good stuff, and a peak (the peak?) of the series as a whole, one of the few times that everything comes together with such intensity and sincerity that it actually works... and then everything falls apart.
It's actually understandable that it did, and surprising that it didn't happen more obviously or more horrifically; Claremont and Byrne were forced to redo #137 after it'd been completed, because the original plan of leaving Jean alive with depowered wasn't thought to be enough after she'd destroyed a planet as Phoenix (FWIW, I think it was a change for the better), but even if they hadn't been, where do you go after a story so cosmic and... well, big? It's no wonder that the majority of the next year (all the way up to the subplots starting in #147, even if the A-plots remained weak until #150) seemed so generic and pedestrian in comparison: After saving the universe from one of their own gone bad, visiting Alpha Flight in Canada to go after a Hulk villain (Even one with Wolverine history) or taking on Doctor Doom and Arcade just doesn't really seem as interesting.
(There's a two-issue exception, of course, the "Days Of Future Past"/" Mind Out Of Time" story in #141-142 that would, once Claremont had exorcized his Phoenix demons, come to define the X-Men franchise with its dystopian, never-smile-because-you're-hated-and-by-the-way-your-future-duplicate-is-more-depressed-than-you-about-it vision. In the context of what followed its initial publication, though, it just seemed like a two-part story without a lot of impact. It'd take a few years to get full-on-depressathon.)
(The ghost of Phoenix haunted the book in more ways than one; she makes a hallucination-appearance in #144, and then the cover of #147 shows an out-of-control Storm with the tagline "We did it before -- Dare we do it again?" It's hard to know whether Scott Summers or Chris Claremont was most affected by Jean Grey's death.)
The loss of Byrne hits the book hard, too; looking back, I still think he and Austen lacked a lot of the personality of Cockrum's earlier issues, but the Cockrum that returns to replace him is a different artist, one who's more conservative and lacking the verve and invention that Byrne papered over with glamor (He's not helped by Joe Rubenstein's inks, either; Rubenstein tends to flatten out a lot of the pencilers he works with, giving everything a kind of generic quality that makes him perfect for a multi-artist project like The Official Handbook Of The Marvel Universe, but not something where you want someone to follow Byrne and Austen.
As the series approached #150, it seemed to have flamed out. With the big villain hinted at for the anniversary issue Magneto yet again, capping off a year of familiar (and non-traditionally-X-Men) villains, it'd wouldn't have been too surprising if fans following the series then were wondering if the series' best days were behind it. Oh, how right/wrong they were.