First of all, Luke dies, so if you were dissatisfied by how The Last Jedi handled his character you were out of luck because the only role he could play in the next film was a voice from beyond the grave. This is particularly galling because the story would have played out exactly the same even if Luke hadn't died of a lonely heart at the end, especially since Rian Johnson easily could have kept Luke alive by simply cutting the shot before his body faded into the Force. If Luke had lived, people would still be upset at his portrayal, but the backlash would have been much more subdued because he still would have had a whole additional movie left to redeem himself.
Then there is the degree to which Luke is out of character. In Return of the Jedi, he risks everything to save the most evil man in the galaxy, then chronologically the next time we see him he's contemplating murdering his sleeping nephew based on a mutable vision of an uncertain future ("Always in motion is the future"—has Rian Johnson seen Star Wars?). The real Luke would have taken this as a warning from the Force that he needed to move quickly to save his nephew’s soul and avert this outcome, not that he needed to kill him in his sleep.
TLJ fans like to say "it was just a moment of weakness, Luke has always been impulsive, he didn't actually attack Ben he just thought about it!" but sneaking into a young family member's bedroom at night and standing over them with a drawn weapon seems like a pretty fucking big moment of weakness. Plus, Luke already faced the temptation to give in to his emotions and kill an unarmed foe in ROTJ and he overcame it. To see him face the same temptation again defeats the point of that moment in Luke's character growth.
People will defend this scene by saying “Oh so once you overcome a certain temptation you can never be tempted by it again?” To which I reply sure you can, in real life. We’re talking about fiction. How is the story of Star Wars improved by hitting the reset button on Luke’s character development so the screenwriter has an excuse to write him as a curmudgeonly sadsack?
I think that a lot of people could live with that one out-of-character moment, however, if it wasn't followed up with an even bigger out-of-character moment, by which I mean out-of-character several years. Maybe some people wanted Luke to be a flawless hero, but I think most would be accepting of him making mistakes if he dealt with those mistakes in a way that followed logically from the character he was in the original trilogy. For instance, Luke only knew Vader as a monster but still believed he could be redeemed and Vader ultimately justified that faith, so it destroys believability that Luke would immediately give up on Ben, the only child of his sister and best friend, a kid he'd known since birth, without even trying to save him.
The basic plot and backstory of TLJ could have stayed the same, but if, following the destruction of his Jedi academy, Luke had gone after Ben and tried to redeem him, or tried to stop Snoke and end his influence over Ben, or gone to Ach-Choo to consult the Sacred Jedi Texts on a way to accomplish either of those things (or even something totally off the wall, like looking for a Force technique to travel back in time and change the past or channel the spirits of all the Jedi to kill the bad guy)—really just done something, anything, other than instantly giving up and running away and abandoning everyone who cared about him—and if he had failed to correct his mistake and only then given in to despair and gone into self-imposed exile, the whole story would be a lot easier to stomach, because then Luke Skywalker, the guy who always tries to do the right thing, at least would have fucking tried to do the right thing.
Not to mention how the way things play out in The Last Jedi makes a mess of what we were told in The Force Awakens. Han says Luke was looking for the first Jedi temple, but he wasn't really. He didn't care about the temple or the texts, didn’t even read them despite being there for years with nothing else to do—he just wanted a place where he could wait around to die of old age. If he wanted nothing more to do with the Jedi, why did he specifically choose the planet that they originated from? There's no reason for it. And how did Han even know about the temple if Luke wasn't actually looking for it in the first place? Also how does the map from TFA fit in? Getting that thing was the main plot of the whole last movie and they don't even mention it or where it came from or who made it or why Father Merrin had it or how it leads to Luke.
There's also the matter of how TLJ's depiction of Luke tarnishes both the legacy of his character and that of the OT as a whole. Luke throwing away his lightsaber and declaring himself a Jedi marked the literal "Return of the Jedi": Vader had destroyed them, but now Vader's son would correct his father's mistakes and restore the Jedi Knights, "passing on what he had learned" and returning peace and justice to the galaxy. That didn't work out so well though. The Jedi never actually returned and Luke died, so the title of Episode VI is essentially meaningless now.
Luke tells Kylo "I will not be the last Jedi," referring to Rey, but from a storytelling perspective, why is Rey better suited to restore the Jedi than Luke? Luke was the original hero of Star Wars. By refusing to follow Obi-Wan's and Yoda's directives to kill Vader, he rejected the dogma of the old Jedi Order and proved himself greater than his teachers. The implication was that the new Jedi would be trained according to Luke's philosophy of love and forgiveness, in direct opposition to the old Jedi’s tenet of non-attachment, and that implication existed unchallenged for over 30 years (our time).
Luke didn't even teach Rey anything, so whatever Jedi she trains will have no connection to Luke's legacy and the whole point of Return of the Jedi. Instead they'll be taught using the Sacred Jedi Texts, which the audience has no emotional investment in and which were presumably responsible for the prequel Jedi being cloistered, emotionless patsies who were bamboozled by a Sith Lord and tricked into facilitating their own downfall.
The movie directly comments on this when Luke declares that "it's time for the Jedi to end" because their sheer fucking hubris led to the rise of the Empire. It's like Rian Johnson saw the prequels, missed the point entirely, and decided there was some fundamental flaw with the Jedi as a concept that would cause Luke to make the same mistakes the prequel Jedi made, as if Luke hadn't already repudiated their failings in ROTJ. Luke is reduced to functioning as a mouthpiece for a screenwriter who doesn't even understand the subpar movies he's trying to critique.
(As a side note, does anyone else think it's really weird and immersion-breaking that Luke would call the Emperor "Darth Sidious"? How would he even know Palpatine's secret alter ego that he hadn't used since the Clone Wars? Why would he refer to the man he only ever called "the Emperor" and whom the galaxy only knew as Emperor Palpatine by an obscure religious alias in the first place? Why would he think Rey would know what the fuck he was taking about?)
Finally, regarding Luke redeeming himself at the end, that might carry more weight if the movie fixing his character wasn't also the one that broke it in the first place. The shot of him fading away beneath the twin suns is very poignant but it's somewhat deflated by the actual details of his sacrifice. First of all, he doesn't even show up to help in person, despite having the means to do so (don't tell me his X-wing doesn't work, that's never established anywhere in the film and the shot of it underwater is clearly there so the audience will remember it later and think that's how he got to Crait at the end).
Obviously the real reason this happens is for the dramatic twist when Kylo and the audience realize that Luke isn't really there, but within the logic of the story it's not clear why he didn't just come in person, or at least fly his X-wing close enough to allow him to pull the same stunt from a distance that wouldn't kill him. So despite the coolness of what Luke actually does, the scene comes across as contrived because you can see the screenwriter's will coming through and overriding the character's.
Not to mention how Luke's execution of this diversionary tactic makes no sense at all, since he doesn't tell anyone that he's buying them time to escape. Poe has to figure it out by himself, but the logic he uses to do so is faulty. He doesn't know that Luke is a projection so he assumes there must be another way out of the base that Luke used to come in. Luke didn't actually do that, though, so it's a very fortunate coincidence that there did in fact turn out to be another entrance. It's not clear if Luke even knew about that entrance, so the audience starts questioning what Luke's plan to save the Resistance even was. Did he just trust that the Force would reveal an escape route to them, and that they would realize he was creating a diversion, and that they would come to both conclusions by themselves? The only reason he didn't tell them is so the revelation that he was a doppelgänger would be a surprise to the audience; in-universe it just creates questions about what his intentions were.
And this is a minor point but it's also strange how Luke's last stand on Crait apparently grew to become a legend across the galaxy, judging by the epilogue with Broomboy and his friends. Luke's exploits during the OT were a much bigger deal, yet by the time of the sequels they had faded into myth, to the point that Rey didn't believe Luke even existed. Somehow, though, Luke's brief reappearance on Crait is known even to stablehands on the other side of the galaxy immediately after it happens. There are only twelve people left in the Resistance, so who is spreading this story? The stormtroopers? From their perspective, some old guy somehow survived at AT-AT bombardment, fought Kylo Ren for a few seconds, then vanished. It was weird, sure, but is that the kind of story that would spread across the galaxy like wildfire? Where are stormtroopers even gossiping with the regular galactic populace?
For my part, the first time I saw the movie I didn't realize that the kids were reenacting Crait specifically. I thought they were just playacting The Adventures of Luke Skywalker, Galactic Hero, and that it was a sweet tribute to the decades of fun Luke brought to kids both in the Star Wars universe and our own. Then when I realized they only cared about what Luke did on Crait and probably didn't even know about his adventures in the OT I just rolled my eyes.
All that being said, I'm glad that people can find things to like in a movie I think is not very good. My pet peeve, however, is when fans are outright dismissive of the issues people have with how Luke was written instead of acknowledging that they exist but admitting that those issues are just not important to them and their enjoyment of the movie. I've seen people say that they grew up watching Star Wars and Luke Skywalker was their childhood hero and TLJ is now their favorite movie in the saga. More power to you if that's the case, but let me know what kind of deathsticks you're smoking to square that circle.
