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 first series:
Author's Saving Throw: When Rob Liefeld quit the title, Fabian Nicieza famously salvaged the team by writing out Cable and bringing exiled New Mutants Sunspot and Rictor back to the team, as well as New Mutants supporting cast member Lila Cheney, as well as shifting the tone away from the grim and gritty to focus on team interaction amongst the core New Mutants holdovers. Furthermore, Cable came back to the team, less angsty and more stern but lovable father figure-like, Feral was written out, and Shatterstar was given a crush on Rictor that gave him more depth than being a generic warrior dude.
Ho Yay: Rictor and Shatterstar. Recent events have made them a fully canonical couple.
The Scrappy: Feral. It's honestly hard to imagine why her teammates kept her around.
The second series / X-Statix:
Ensemble Darkhorse: Blink gained quite a fan following after a fairly short-lived existence in their mainstream storylines. She (or rather, her Alternate Continuity counterpart) got an upgraded role in the Age of Apocalypse storyline. She was so popular that she survived the destruction of that universe and became a main character in the Exiles spinoff (though it took five years).
Edith "Edie" Sawyer/U-Go Girl: A fan favorite. This is probably why she was killed off.
Wangst: Guy Smith/Orphan, aka Mister Sensitive. Oh, the wangst...
The third series:
Complete Monster: This incarnation of X-Force was put together for the purpose of wiping these off the face of the Earth. The Purifiers, Reverend Craig, Selene, etc. Arguably the only one to whom this doesn't apply is Eli Bard. He's just a guy gone completely insane over his miserable eternal life and desperate for someone to love him. Unfortunately the only way to get the one he loves to love him back is to kill millions of people. Which he doesn't have a problem with anymore thanks to a few centuries of crushing loneliness driving him ALL the way round the bend.
Crosses the Line Twice: Elixir, a much kinder and more peace-loving member of the team, gives Vanisher a fatal brain tumour? Harsh and a little bit depressing. The revelation that he made it the shape of the X-Men logo? Hilarious.
Fridge Brilliance: Why did Weapon Infinity go after Fantomex, who killed Apocalypse, when he removed the one thing standing in their way of omni-versal domination? Because he's making a new Apocalypse.
Heartwarming Moment: Nightcrawler asks Deadpool why he'd go on such a dangerous mission, especially since he lost his healing factor. His answer (well, second answer) was that he didn't want to let anyone down, because the X-Force is the closest thing he's ever had to a family. Remember, this is coming from the guy who's been mistreated pretty much his entire life, and has only ever had a handful of friends.
Later turns out to be invoked. Fantomex knew Psylocke didn't really have feelings for him, but he had to make her question her devotion to Warren in order to acquire the courage needed to kill him.
Future!Wolverine and Future!Psylocke are together.
Unpopular Popular Character: Deadpool is tolerated at best by the rest of the team and actively despised by Wolverine. Most egregious example: when Fantomex shocks the team by killing kid!Apocalypse in cold blood, Deadpool takes the brunt of the team's wrath for openly questioning the killing. Leads to Dude, Where's My Respect?.
Deadpool does garner a little more respect from his teammates after they've been through a few life and death situations, especially Wolverine and Fantomex, oddly enough.
Cable and The X-Force
The Scrappy: Hope Spaulding, Bratty Half-Pint and former Mutant Messiah. Part of this is due to negative feelings regarding her role in the resolution of Avengers vs. X-Men where she eventually sides with the Avengers in taking down Dark Phoenix Cyclops, giving her her the same image of race traitor and Avenger flunky that Wolverine and Havok also share. The Other part comes from the existence of Teen Jean in All-New X-Men making Hope's other point as a character meaningless.
Strawman Has a Point: The first arc involves the head of a Chick-Fil-A stand-in who bars mutants from eating in her establishments. When confronted, not only does she explain that her daughter was killed during Xorn's attack on NYC, but also points out that superhumans tend to cause insane amounts of collateral damage wherever they go. Thus, her desire to not see her customers and employees killed comes off looking pretty rational, all things considered.
Tough Act to Follow: Replacing not only one but two X-force series, which both ended without proper send-off for the teams, the new series wasn't recieved very warmly. It also gained many negative reviews for it's artwork and different writing.
Issue #5 - Marrow undergo Volga's repowering treatment willingly while pregnant well aware it might make her miscarriage. Which happened.
Issue #6 - we find out what happened at Alexandria, why Hope is in coma an that Cable is dying from Murituri formula and using clones to prolong his life and then we find out that Meme is dead and has been dead since her first apperance. Meme we know is really Hope using her powers.
Issue #7 - Fantomex secretly kills Cable's clones every day to overcome his inferior complex. Domino is captured and has to witness the clone's not knowing it is one, before being put into a mindcontroll system.
Ensemble Darkhorse: Clarice Ferguson/Blink. Blink gained quite a fan following after a fairly short-lived existence in their mainstream storylines. She (or rather, her Alternate Continuity counterpart) got an upgraded role in the Age of Apocalypse storyline. She was so popular that she survived the destruction of that universe and became a main character in the Exiles spinoff (though it took five years).