The Overlook's name. Presumably the name was given in reference to the hotel's spectacular view overlooking the mountains...but the other, more common meaning for "overlook" is "to fail to notice something." The hotel is destroyed because Jack overlooks the defective boiler.
Also worth a mention: Jack, Wendy and Danny have been there all winter and they would've spent their spare time checking out the hotel. It seems that they also overlooked Jack's appearance in the ballroom photograph marked July 4th 1921.
The difference in narrative tone in this book and its sequel Doctor Sleep can be attributed to Early Installment Weirdness, but actually makes much sense. The first book is centered around Jack, who is a intelligent writer and avid reader, explaining its tighter and more literary prose. However the sequel focuses more on his son Danny who is neither and more relaxed, explaining its more casual narrative.
Writers Cannot Do Math: Wendy tells the doctor that Danny's shoulder had been dislocated six months earlier. Later, while bitching to Lloyd the Bartender, Jack says that the dislocated shoulder incident had happened three years earlier. This becomes some very creepy Fridge Horror if you think about how Wendy specifically phrased it: the time of the accident isn't specified, only that Jack immediately afterward promised not to have another drink in case he hurt her or Danny again. So there's a period of two and a half years during which Jack must have done something to break his promise, which Wendy clearly doesn't want to tell the doctor about.
Jack also mentions that he hasn't had a drink of alcohol in FIVE months, which seems to indicate that there was about a month in between the time where Jack promised to stop drinking and the incident with Danny. Implies a good few weeks of uncomfortable nights in the Torrance household, if nothing else.
In the book, Jack didn't actually stop drinking until a DUI incident with his friend Al, who was the one driving. The two ran over a bicycle that had been left in the middle of the road, and despite there being seemingly no owner around, the incident freaked both of them out enough to get on the wagon.
Remember how attached Ullman was to the Overlook in the book? Imagine what his reaction will be when he finds out about its destruction.
When Wendy sees Dick's body on the floor, she's so freaked out and has seen so much nightmarish shit that there's no reason to think she realizes she's seeing a real person and not another ghost. This is especially unnerving if you think that it takes a few hours to bleed to death.
The Grady Twins' corpses show that they were killed in a dead end hallway: their father cornered them before massacring them.
It's easy to picture Grady pursuing his terrified children very similarly to Jack chasing Danny through the kitchen. But for the Grady girls, there was no escape.
Jack and Danny both show signs of "dissociation;" Danny with his talking finger, and Jack's conversations with the ghosts. Dissociative Identity Disorder isn't the kind of disorder that passes down genetically; it's caused by abuse early in childhood—most commonly sexual abuse.
Observant fans of the film have noted the creepy scene where Danny asks Jack, "You'd never do anything to hurt me or Mommy, would you?" The scene abruptly ends, and the next time we see Danny, "Tony" is telling Wendy "Danny's gone away Mrs. Torrance." The scene with the man in the bear/dog suit is often considered a symbol of Danny being molested by Jack, and Wendy finally consciously acknowledging what she's known in the back of her mind all along.
With this in mind, how did Jack become the psycho he grew up to be, and what will Danny be like in a few decades? How long has this cycle of abuse and resulting psychological problems being going on in this family, and how long will it continue?
King thankfully answered this with a happy ending in the sequel to the book, Doctor Sleep. Danny did inherit his father's alcoholism and temper, and suffered for it, but he was ultimately able to prevent himself from repeating history all the way, and turns out to be a positive father figure to the young heroine, his niece Abra.