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 an 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.
It is when Jack murders Dick with the axe that the Native American chanting and/or screaming begins, and Wendy starts to also see the ghosts and visions of the Overlook. Though Dick's "shining" was nowhere near as nutritious for the Overlook as Danny's would have been, it was still enough to strengthen it to the point that it could inflict the same horrors on her as it inflicted on Danny and (presumably) Dick.
Not quite- the musical piece playing there is the Resurrection from Penderecki's "Utrenja: Ewangelia", and it's in Polish(!). Though Kubrick and Carlos undoubtedly picked it because it sounds terrifying, the chorus are actually declaring the resurrection of Jesus Christ!
During his talk with Dick, Danny remembers a time when Tony showed him a vision of a baby, implied to have been a possible sibling of his, which didn't come true. In the same conversation Dick confirms that the Shining isn't infallible. But come Doctor Sleep thirty years later, we find out that Danny does have a half-sibling, Lucia, so he may well have been right about the baby, just mistaken in thinking it was his mother's.
Crosses a bit with Fridge Horror, but the True Knot mentions that children with the "shining" were getting fewer. It's already been shown that the "shine" seems to be genetic and gets stronger with each generation as seen with the progression from Jack, significant enough to be influenced by the hotel but not much else, to Abra, the strongest seen by the True Knot in a long time. It makes sense why the magic seems to be going away if they're killing kids before they can grow up and have kids with the gift. It's basically killing off a mutation before it can spread to the population.
Behind all of the ghosts, the central horror of The Shining film is the fracturing of a family due to abuse and addiction taking over. It's notable, then, that for all the ghosts that appear in various locations of the hotel, there's an exception. In the Torrance living quarters, the only monster ever presented there is Jack—first when Danny fearfully tries to retrieve a toy truck, and then at the climax where he tries to kill his wife and son in the apartment. The broken-home horror is unpeeled at its most frank and least supernatural when played in the hotel's most home-like setting.
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.
Speaking of this particular incident, the sequel novel Doctor Sleep and the M.O. of the antagonists, the True Knot, lends its own brand of horror to it. The True Knot are shown to have no problems with abducting their victims straight from the road, as happened in Iowa in the case of Bradley Trevor, the "Baseball Boy." What if Jack and Al had happened to come across the aftermath of one such abduction? This in turn yields an extra bit of horror when one realizes how close they would have been to Danny in this case, and that only a miracle prevented them from sensing him in turn and going after him.
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.
