These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The Magus is probably among the most powerful foes any X-team has ever faced, and the New Mutants squad, the Professor, and a cosmically-powered Carol Danvers ended up stopping him. And Doug Ramsey was the guy who won the battle.
Crowning Moment of Heartwarming: Nate Grey is getting those in new series, first with Cyclops assuring him he is finally home and second with Hope Summers taking him under her wings, despite all issues she must have with his pressence.
Ensemble Darkhorse: Rahne has been a member of more X-Men spinoff teams than any other character.
Fetish Fuel : Selene and her mind control could be used quite erotically; when she forces Mirage to enjoy the sacrifices, for example.
Empath's powers and the way he uses them. He has absolutely no qualms about manipulating another person's emotions against their will all because he thinks it's amusing.
The Smiley Faces.
Ho Yay: Arguably Warlock towards Cypher before the latter's killing-off.
Still present after Cypher came back. "I love you and you are breaking my heart." Well...
Between Shatterstar and Rictor as well
Les Yay: Danielle and Rahne, due to their psychic-link when Rahne's in wolf-form.
-and a good deal of obvious subtext, calling each other 'soul-mate', a revealing event in the Inferno-alternate Manhattan, and Dani saying as much as 'I love y-' on a cliff in Scotland.
Sometimes, Illyana towards Kitty, too.
Moral Event Horizon: Pierce crosses this several times in the New Mutants origin story alone. Examples are ordering the death of Dani's grandfather and having his men trying to kill each of the New Mutants who were, at the time, teenagers who had only just discovered their powers.
The Ani-Mator also crossed this by killing Cypher.
Bird Boy is probably the most extreme, being an imbecilic half-animal who derailed the book into whacky antics and over-eating. He's actually one of the most-accurate in-continuity versions of the Jackovasaurs from South Park. At least he was rapidly Put on a Bus.
An entire team of Scrappies was folded in following the Inferno event with the X-Terminators: Boom Boom, Rusty, Skids, Arty, Leech, Rictor, Whiz Kid. Luckily the bulk of the characters were quickly put on various buses, leaving Rictor and Boom Boom to become beloved members of the group, though this derailed the book from a thoughtful angsty series into the very "mall-going brats" adventures that old-school fans thanked the old writers for not using.
They Changed It, Now It Sucks: The level of "dumbing down" from the earliest issues to the time of Cable's leadership is so marked that it also marks the beginning of the Liefeld Age of comics; chrome arms, pouches, guns, pointless angst, no-character writing, teeth-gritted action, "ninjas appear from the future", etc.
They Wasted a Perfectly Good Plot: Cypher was a polarizing character amongst fans of the title in the 1980s, in part because he had sensible everyday mutant powers in the form of the ability to understand any form of language, which sadly also made him utterly useless in fight scenes unless he merged with the alien Warlock. So when Chris Claremont left the book and Louis Simonson took over, Simonson decided to kill off Cypher since she didn't feel like coming up with excuses for why he was MIA in fight scenes, plus felt that no one liked him. Sadly, when she killed him off, fans revolted against the death. Even the fans who were calling for Doug to be killed off! And to this day still fans rant and rave about how Cypher had so much potential that was utterly wasted by Simonson misreading the room.
It's a real shame that Doug was killed before the internet became near-universal or he could have been the world's greatest hacker.
Cable versus Apocalypse/The Twelve, which was the replacement for the Highlord Ascension storyline when it was dropped, was haphazardly aborted for a lame-ass month-long event called "Ages of Apocalypse" involving the X-Men in different time periods play-acting various scenarios.
Tear Jerker: The issue that dealt with Ilyana's death - or, rather, Jubilee's reaction to it. Even the knowledge of Comic Book Death and the likelihood of Ilyana's eventual return wasn't enough to dampen the emotional weight of a tale about a little girl slowly dying as the adults around her slowly come to the realization that there's nothing they can do to help. Although "I won't allow her to be placed into a living death!" isn't a good argument when it might have allowed them time to find a healer and it certainly would have allowed time for her brother to get home before she died.
Cult Classic: The DeFilippis/Weir run. The later Kyle/Yost run has a more traditional sort of fanbase.
Fan Preferred Couple: Elixir/Wither, for both the obvious and the fact that their powers cancel each other. But mostly the obvious. Wither is also more commonly paired with Mercury than his canon crush, Wallflower.
Internet Backdraft: Whoo boy, DeFilippis/Weir vs. Kyle/Yost. DeFilippis has been known to defend Kyle and Yost from angry fans, for what it's worth.
Sympathetic Sue: A candidate for the most messed-up backstory of any X-character.
The Scrappy: Icarus. You'd think this would be because of acting like a moron and inadvertently getting himself and half his friends killed in the first Kyle/Yost arc... except nobody much liked him under DeFilippis and Weir, either. Perhaps he still had the stench of the infamous Chuck Austen Uncanny run (the first run to prominently feature him, though he'd been a background Guthrie child in older comics) about him?