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.
They Changed It, Now It Sucks: A lot of "Liefeld-style"(angst, anger, guns, and innuendo)X-Force fans refused to have anything to do with X-Statix.
Wangst: Guy Smith/The Orphan, aka Mr. 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.
Sabretooth. While this is hardly out of character for him, he sinks to new levels of despicable pettiness when he reveals he started the entire Brotherhood scheme purely as an excuse to Troll Wolverine into killing another of his own children, not caring about getting many of his comrades killed (not to mention all the emotional damage to X-Force themselves) in the slightest.
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. lolwut?
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.
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).