The Dark Tower provides examples of the following tropes:
Affably Evil: Some of the Devar-Toi guards, especially Trampas, are pretty nice and seem like good guys, it's easy to forget that their job is to essentially destroy the universe.
Alas, Poor Villain: After Mordred kills Flagg/Walter/Marten, finally eliminating one of King's great villains, the narration briefly runs through Flagg's life. It gives him a Freudian Excuse (he was raped as a teenager) and manages to wring some small amount of sympathy from Flagg's death.
The death of Trampas, one of the Mooks guarding the Devar-Toi, is rather sad. He works for the Big Bad, but he's actually a pretty decent guy once you get to know him. It's made pretty clear that Ted really doesn't want to kill him and even yells at him to get out of their way, although he is forced to eventually resort to throwing a mind-spear at him, killing Trampas in the process.
Angrish: The Crimson King keeps screaming "EEEEEEE!" in wordless rage.
Eat the Dog: When crossing a frozen wasteland, Susannah is so cold that she eventually considers killing Oy for his fur.
Enfante Terrible: Mordred, until he turns into a teenager about 2 thirds into the story anyway.
Happily Ever After:"Will I tell you that these three lived happily ever after? I will not, for no one ever does. But there was happiness. And they did live."
Hell Is That Noise: invoked The chewing sounds made by something behind a door with an undecipherable symbol. Also, the todash chimes.
Last Breath Bullet: Eddie is killed by the Devar-Toi's warden, Pimli Prentiss, doing this.
Late Arrival Spoiler: The ending is highly talked about, which may prove unfortunate for many prospective readers.
Never My Fault: Ted is livid when he sees a Breaker child dying during the attack on the Devar-Toi and it's implied that this is part of the reason why he kills Trampas. Nevermind that the child wouldn't have died if he and Roland's group didn't decide to set some buildings on fire to cause a stir.
New Game+: In the next Dark Tower cycle Roland gets to keep the Horn of Eld. This might be meaningful in itself, as Roland is drawn to the tune of the Dark Tower. Now that Roland has an instrument of his own he can make his own music and be free from the Tower forever. The series is based in part on Robert Browning's poem "Childe Roland to the Dark Tower Came", which ends "Dauntless the slug-horn to my lips I set, and blew "Childe Roland to the Dark Tower Came", a further inferrable suggestion that having the horn will make the difference. It may also be a nod to Roland's almost inhuman nature near the start of the books — he has no emotions, only the Tower. Following this version of his quest, he learns some of the values of family and love and therefore spares the time to take the Horn of Eld from his dead friend's body.
The Tunnel Demon that goes after Roland, Susannah, and Oy down "Main Street". While they can't see it, the mere thump noises are enough to convince them to run. It is equally terrifying when they do see it.
Pragmatic Villainy: Prentiss treats the Breakers well and wants his subordinates to do the same. Not because he particulary cares about them (though to be fair, he is shown to be sympathetic towards them in a few occasions), but because they're less likely to revolt or try to escape if they're happy and well.
Rage Against the Author: The villains want King dead. And while the heroes have to protect him, they're not happy about it at all.
Reset Button: Roland is forced to restart his quest. The reasons for this are unclear, but it's implied that his willingness to abandon his friends to reach the tower is part of the reason why. That he didn't have the Horn of Eld with him could be a reason as well. It's implied that he's had to do this many times before because he never got it right. The afterword by King implies that he might get it right the next time.
Sympathy for the Devil: Mordred is occasionally described with sympathy as his situation becomes increasingly pathetic. A few times he's described as an abandoned child.
Look, if you would. Here sits a baby with blood streaking his fair skin. Here sits a baby weeping his silent, eerie tears. Here sits a baby that knows both too much and too little, and although we must keep our fingers away from his mouth (he snaps, this one; snaps like a baby crocodile), we are allowed to pity him a little.
Well-Intentioned Extremist: Prentiss genuinely believes that ending the universe is the right thing to do, as he thinks a new improved universe will take the old one's place. Some of the other Devar-Toi guards are implied to think so as well.