After the End: The boys were on a plane fleeing the soon-to-be-nuked UK. Though the home country is apparently in dire straits, the British Navy is still out doing its job as best as possible. May have verged into Cosy Catastrophe if it wasn't for the fact that mention of the war was a very minor framing device with little direct bearing on the story.
Artistic License - Physics: The divergent lens of Piggy's glasses (required for his short-sightedness) would be useless for lighting a fire. You would need a convergent lens, found in reading glasses. The kids would have had to learn one of the other methods for starting fires that they discussed, but that would break the plot.
It's easy to assume Piggy is farsighted until, at the beginning of chapter eleven, it says: "Piggy sat expressionless behind the luminous wall of his myopia." The concave lenses needed to fix myopia are incapable of focusing light to a point, thus incapable of starting a fire. Whether Golding was unaware of that distinction or had myopia and hyperopia confused, the net result is a clear-cut case of poor research.
Beauty Equals Goodness: In the book, The Hero Ralph is described as fair-haired, athletic, charismatic and handsome; Rival Turned Evil Jack is redhaired, with freckles and ugly. The movie averts this. Jack is a perfectly pleasant looking boy.
Deus ex Machina: Ralph is rescued by the Royal Navy at the last minute, effectively out of the blue. This was intentional, as the Naval Officer is important in rounding off the themes of the story, and the sudden abruptness with which he appears (with absolutely no foreshadowing) is crucial to highlighting the rapidity with which the appearance of an authority can change everything.
Dewey Defeats Truman: Lord of the Flies is actually set in the near future (of 1956), after a nuclear war between the USSR and Britain. This is mentioned by some of the boys in the book (along with the existence of nuclear bombs and their use), although usually in passing.
Foreshadowing: In hindsight, the arrival of the Royal Navy at the book's climax is predicted early in the book, though none present could have guessed the circumstances under which that would happen.
Four-Temperament Ensemble: Ralph and Jack are Choleric, Piggy is Melancholic, Samneric are Sanguine, and Simon is Phlegmatic.
Freudian Trio: Piggy: the superego; Ralph: the ego; Jack: the id.
Friend to All Living Things: Simon is the only boy who enjoys the island as a natural paradise and adopts a lizard as pet. Even, he makes a little "Happy Place" to himself in the more nature-beautiful side of the woods that represents his innate good nature and innocence. The sow's head placed later represents how evil has started to contaminate.
Genre Savvy: Ralph describes being on an island "Like a book." Which causes many of the littluns to name out titles such as "Treasure Island" and similar stories.
Good Is Impotent: A major theme of the book. Raplh and Piggy can't get the boys to work together for their own good when faced against the savage impulses that Jack champions, Simon, The Heart, spends all his time alone until he has an epiphany about "the monster" and is unable to tell the others about it, and in subtle but telling passage, when the twins, the last boys who support Ralph and Piggy, are bound by Jack and the others, the text describes their meager protests as "protested out of the heart of civilization" with lines like "Oh, I say!" and the like. This is contrasted with the brutal directnesss and action of Jack and his followers, and how easily it overwhelms the good.
He Who Fights Monsters: Jack and his choir and eventually almost every boy on the island turn into savage beasts in their obsession with killing the beast.
Hobbes Was Right: Could be considered to be an example of this trope, but it could also be considered to simply be an example of a Crapsack World setting implying that government and anarchy are both bound to fail given that it starts with a wartime evacuation and ends with the boys being rescued by a naval warship, thereby subtly implying that even "civilised" society is not actually any better. The book is accessible to several critical interpretations.
Humanoid Abomination: The pig's head on a stick is suspiciously human-like, especially at a distance. It's about the height of a person... only with a pig's head instead of a human head. Considering it was quite dark when the boys first put it up, no wonder they ran away.
"Kill the beast! Cut his throat! Spill his blood!"
Meaningful Name: Three of the main characters are named directly after Coral Island characters. The fourth? Simon from Peterkin.
Messianic Archetype: Subverted and deconstructed. The description of Simon's death sounds very much alike a atonement sacrifice, but his death rather than save the boys for their sins, or just show them that how they act is wrong, instead it just push them deeper into moral guilt as they proceed to completely forget about him cementing the lost and oblivion of innoccence.
Nice Job Fixing It, Villain: Jack has the entire island on fire to kill Ralph - the massive amounts of smoke actually summon a Naval ship, which Ralph and Piggy had been awaiting the whole book long.
No Good Deed Goes Unpunished: In order to soften the blow of losing his leadership to him, Ralph declares Jack leader of the hunters. This earnest gesture can largely be seen as the beginning of the end, as it inadvertently sows the seeds of a rival faction with Jack at its head.
Non-Action Guy: Piggy has asthma, and Simon suffers fainting spells, so both qualify.
Superego: Piggy (or Simon, if you think that makes more sense). Actually one could argue that Piggy is also the id. He represents the societal conscience but when it comes to personal desires, like food, Piggy is the id.
Rule of Symbolism: Most of the book. Jack insists on being called "Merridew" and leads a choir. Eventually, after he discovers how to kill, his group degenerates into savagery. Sound similar to the story of anyone we know? Oh, and the spectral corpse on the mountain that terrifies everyone is the Shadow of War, harmless in fact (the pilot is dead), but terrifying to look at.
Satan: "Lord of the Flies" is a literal translation of the Hebrew "Baalzevuv", root of the modern "Beelzebub". note The ancient Philistines worshiped the lightning god Ba'al, referring to him as "Ba'al Zebūb", or "Lord of Zebūb". "Ba'al Zebūb" sounds very close to "Lord of the Flies" in Hebrew. The ancient Israelites used this fact to mock their enemies.
Shout-Out: Golding wrote Lord of the Flies partly to subvert children's adventure literature, in particular The Coral Island by R.M. Ballantyne, which also provided the names of Ralph and Jack. The Naval officer even has the following line:
"I know. Jolly good show. Like the Coral Island."
This completes the Book Ends begun when the children first realize they're alone on a tropical island.
Survival Mantra: Poor Percival, one of the littluns' having been driven to a depression and loneliness from the neglect by the older boys tirelessly recites his name and address—"Percival Wemys Madison, of the Vicarage, Harcourt St. Anthony"—like an incantation", only for his memory to begin to falter as the story and degradation of the island's civilization progresses. By the end, when Percival is finally able to recite this crucial information to someone that can actually help him, he can only mouth wordlessly at the naval officer, his time on the island causing him to have forgotten it.