Is ka truly "destiny", or is it Roland misidentifying his surfacing memories of his past times through the time loop as such?
The Filk Song "Terror Train" by Demons & Wizards portrays Blaine as a pathetic Psychopathic Manchild with an Inferiority Superiority Complex, who is still a legitimate threat. The chorus: "No one really knows the pain I'm going through (My final ride is almost done!) / The world has moved on, now it's time to go (My final ride is almost done!) / Come lift my spirit up, I'm BLAINE!". Aww, he's lonely! But he's also batshit insane and suicidal, and misery loves company...
Anti-Climax Boss: Just about every important villain gets defeated in a Curb-Stomp Battle, no matter how much they're built up as a threat:
It's made to look like the Tick Tock Man will be the first villain to present an actual physical threat to the gunslingers, being a muscle-bound badass who might actually be faster than Roland himself. Nope, he's defeated by Jake once, then brought back just to get unceremoniously gunned down a second time without ever putting up a fight.
Randall Flagg gets killed by Mordred without putting up any significant resistance. There's even a momentary fake-out where it looks like Flagg might have gained a small advantage, but the narration negates it in the very next sentence.
In spite of getting an entire book to herald his introduction and possessing mind-control powers, Mordred doesn't do much after he's set free. For a long time he just follows behind Roland's crew, starving and freezing. By the time he finally attacks, he's dying from poisoned meat and does nothing but charge at Roland. Although he does manage to kill Oy, he's killed by a few well-placed bullets, just like any other mook.
The Crimson King is some sort of immortal demi-god whose presence is felt through most of the series. By the time we see him, he's just a crazy old man with nothing in his arsenal but sneetches, which we've seen used several times before and present almost no threat to a gunslinger. He's defeated by Patrick.
The ending, where the Crimson King, the main villain, turns out to be just an angry old man with some clever technology and the result of the main characters saving Stephen King is that he writes a boy with magic powers into the story so that that boy can defeat the Crimson King, is extremely controversial. Some fans think it fits in perfectly, others think it ruined the whole series.
The publication of The Wind Through the Keyhole: was King just cashing in/flogging a dead horse/attaching an unrelated fantasy novel to the Dark Tower brand to ensure it would sell? Or was it as on-point as the series' other flashback novel Wizard and Glass, and patched up a small Offscreen Teleportation plot hole between the former book and Wolves of the Calla to boot? Or, to Take a Third Option, was it a perfectly fine King novel in the vein of The Eyes of The Dragon that just didn't need to be so heavily interwoven with the DT series to work?
The Chris Carter Effect: King mentioned early on in the afterward to The Gunslinger that he had no idea where the series was going. By the end of the series, it gets to the point where numerous plot points are ignored, contradicted or handwaved as Canon Discontinuity.
The Crimson King is the demonic offspring of Arthur Eld and the Greater-Scope Villain of Stephen King's multiverse. A sadistic entity who proclaims the glory of chaos, or "The Red", the Crimson King presides over a court of nightmare and slaughter while sowing evil and chaos across the realms. The being behind all the evil and destruction of Randall Flagg; the rampaging armies of John Farson; and agents such as Atropos and Munshun, the Crimson King has the nation of Gilead annihilated and its people slaughtered, while having psychic children known as Breakers taken to have them sucked dry so their powers may be used to help open the path to The Dark Tower. The Crimson King's ultimate goal is to ascend to the top of the Tower and consume all the multiverse, ending everything that exists so the King may rule the primordial chaos that results. At last poisoning his own court to become undead and losing his mind and power, the Crimson King faces Roland while seeking to ascend the Tower and end all reality.
Randall Flagg (real name Walter Padick) has lived for so long and accumulated so much power that he has become the emissary for the Crimson King. Flagg earns Roland Deschain's undying hatred for beating and sleeping with Roland’s mother, and for aiding the revolutionary John Farson in causing the destruction of the city of Gilead. Flagg also forces Roland to let his preteen companion Jake Chambers die, and drives a girl insane by telling her the trigger phrase which causes a formerly dead man to recount the afterlife to her. Corruptive, treacherous and sadistic, Flagg's ultimate goal was to betray his master and climb to the top of the Dark Tower in order to become God of all.
Creator's Pet: Susannah is often cited as the most disliked character of the Ka-Tet, largely due to her split personalities coming across as annoying to many. She's also the only one other than Roland who gets an entire book dedicated to her, and the only one that gets an (arguably) happy ending. Of course, other fans (such as The Losers' Club, a Stephen King-centric podcast), frequently cite her as one of their favorite characters.
Ending Fatigue: The series ends, then gets an epilogue for Susannah's ending, then gets a Coda for the actual ending. There's also an afterword.
Esoteric Happy Ending: Susannah goes into an alternate reality version of New York where Eddie and Jake are still alive and in fact are brothers. She appears in Central Park at Christmas time, alternate-Eddie greets her with a cup of hot chocolate, and it's clearly supposed to be her happy ending... except many readers feel that Susannah abandoned the quest and is now trapped in a world that isn't her own with a couple of Replacement Goldfish who aren't really the people she loved.
The Tunnel Demon in The Dark Tower. A giant albino worm chasing after Roland and Susannah through the tunnels between Fedic and the Badlands for God knows how long. They only survive because it is sensitive to light, but their Sterno (artificial fire) gets notably less and less in time. The light at the end of the tunnel comes as a great relief to both Roland and Susannah and the readers.
The Charlie the Choo Choo book is an in-universe example; King definitely intends for his own readers to find it creepy.
Mordred's birth.
Replacement Scrappy: After being King's ubervillain since the 70s, King thought it was a good idea to replace Randall Flagg with the newly-introduced Mordred as Roland's main antagonist for the last book.
