Genius Loci: Several are implied, but the mountain Caradhras is perhaps the most aware (and malevolent).
Genre Savvy: A lengthy conversation about characters in adventures occurs between Frodo and Sam on the steps of Cirith Ungol. They accept that they are at one of the darkest places in their story, but characters in a similar position in the old tales have overcome such difficulties, so there is still hope.
Sam: "That's not the way of it with the tales that really mattered, or the ones that stay in the mind. Folk seem to have been just landed in them but I expect they had lots of chances, like us, of turning back, only they didn't. And if they had, we shouldn't know, because they'd have been forgotten."
Gentle Giant: Treebeard is as tall as several dozen hobbits stacked on top of each other, yet he tenderly carries Pippin and Merry through the forests for days without expecting them to repay him or benefit him in any way.
Gentleman Adventurer: Bilbo and Frodo. Neither of them had obvious means of support (Though it's implied that they did receive a substantial inheritance, Bilbo from his mother and Frodo from Bilbo), and they lived relatively well. Neither wanted adventure at first, but after some prodding found they had a talent and a taste for it. Contrast this to Frodo's companions Merry and Pippin, who were both heirs to working farm estates, and Sam, who was an actual handyman/laborer. We never find out much about Fatty Bolger's source of income.
Go Ye Heroes, Go and Die: Éomer (Theoden in the movies) to the Rohirrim at Pelennor Fields. Their culture regards keeping their oaths as more important than surviving, so it's a Rousing Speech rather than a demoralizing one.
God in Human Form: The group of so-called "wizards", while appearing as old human men, are actually five Maiar, a kind of angelic spirit, who are themselves incorporeal but can usually clothe themselves in any form they like. The five have been sent on a mission to help the peoples of Middle-Earth against Sauron, during which they are bound in their physical form, unable to change it, and also limited in their powers and knowledge.
God's Hands Are Tied: The Valar are not permitted to interfere more directly, due to events in the Backstory. The Wizards are likewise limited in what they can do and strictly forbidden from fighting Sauron directly because that's not their job.
Gondor Calls for Aid: in the Backstory. The Trope Namer is film-only; Rohan and Gondor are long allies because of a straight play of the trope several hundred years ago (which results in Rohan and Gondor being treaty-bound to act as The Cavalry for each other in the present day) and Theoden answers right away. The people of Gondor have faith in Rohan's relief effort - the tension come from the possibility that the Rohirrim will be too few in number, or arrive too late.
Going to See the Elephant: Sam. He gets to see one, and realizes the Hobbit rhyme is an appallingly realistic depiction of a Múmak.
Good Hurts Evil: Shout A Elbereth, Gilthoniel at a Nazgûl and watch them wince.
Good Is Not Dumb: While they plot to enter Mordor, Frodo explains to Gollum that he is well aware Gollum has treachery in mind. Sam is surprised at this because he had assumed Frodo was far too good a person to be able to understand deception and treachery. Sam admits that he (and probably Gollum as well) "had confused kindness with blindness."
Denethor. He's certainly fighting to save Gondor from Sauron, but he's also a racist against "lesser Men" who treats Gandalf, Pippin, and especially Faramir like crud. What he was like beforeSauron messed up his head via Palantír, we never see.
Frodo, the archetypal Nice Guy, shows signs of this as his responsibility to the world - and the burden of the Ring - grows on him. At one point Gollum begs Frodo to 'go away, go to nice places, and give it to little Smeagol.'
Gandalf, who has a short temper and a caustic sense of humor.
Good Is Not Soft: Sam disapproves of Frodo treating Gollum with mercy, as he believes that Gollum is extremely untrustworthy and Frodo is being too soft with him. It comes as quite a shock to him when Frodo explicitly states he would kill Gollum as a last resort if he betrayed them.
Sam looked at his master with approval, but also with surprise: there was a look on his face and a tone in his voice that he had not known before. It had always been a notion of his that the kindness of dear Mr. Frodo was of such a high degree that it must imply a fair measure of blindness.
The Good, the Bad, and the Evil: There is this dynamic between the main characters. For one side, you have Frodo, or Sam (if one considers Sam an even more pure good character) as the good, then Gollum/Smeagol as the bad (but still sad and pitiable creature) and Sauron as the evil.
Grand Finale: This story marks the end of the Third Age of Middle-Earth and is chronologically the very last installment of Tolkien's Legendarium. (He did begin a story, The New Shadow, set about 220 years into the Fourth Age, but didn't get far with it.)
Morgoth, who is only referred to by Aragorn. The setting's 'Satan' analogue: his defeat at the end of the First Age left him powerless to interfere in the mortal world, long before the book begins.
Green Aesop: Saruman's deforestation of Fangorn and later the Shire is shown to be every bit as heinous as his plot to take over Rohan. One of Tolkien's strongest messages and one of his most beloved and relevant for readers from the 1960s to today.
The Guards Must Be Crazy: Aragorn happily reports at the Council of Elrond that he gave the Mirkwood elves custody of Gollum, figuring they would keep him quite safe. Legolas speaks up, having been sent specifically to report that Gollum escaped. The elves thought he was so pathetic that they gave him some parole outside in the extremely dark, dense forest full of nasty things, which he is good at communicating with, and actually let him climb trees with a guard posted on the ground. (Not surprisingly, Glóin is rather irritated at this liberty, considering what his own stay was like.)
Half-Human Hybrid: Elrond and his family are half-Elven, Aragorn's ancestors had Elven blood (his ancestor was Elrond's brother), and the Uruk-hai are rumoured to be part-human part-orc. If you trace the bloodlines of the Númenóreans back to Lúthien and then look at HER parents, you find that her father was an Elf Lord and her Mother was a Maia. As such, Aragorn, Elrond and company are part spirit/angel as well as elf.
Handicapped Badass: The Nazgûl have miserable eyesight during daylight and are blinded in bright sun. They rely on scent instead, which makes them sniff very loudly when trying to find people.
Hannibal Lecture: Gríma Wormtongue is a student of Saruman's, and earlier uses similar Hannibal Lecture techniques on Théoden to render him helpless and hopeless against Saruman, and on Éowyn in order to break her resolve and drive her to desperation.
Most obvious is the abundant use of 'queer', which is rarely used to describe something strange any more. Likewise 'gay' is used on a couple occasions to mean 'happy'. Some material also falls into Separated by a Common Language, since in parts of the world the term 'faggots' isn't any longer used to represent bundles of wood. Also, though it doesn't come up in the book, Celeborn's alternate name is the hilarious Teleporno (in Tolkien's planned but unpublished retcon of Celeborn's origin, he was not a Sindar Elf but a Teleri Elf; Celeborn is Sindarin for "silver tree", Teleporno is Telerin for the same).
The dwarf Gróin, father of Óin and Glóin, though only if you pronounce it incorrectly (the two vowels have a syllable between them, so it's really Gro-in). The orc Shagrat.
The song that Sam sings near the stone trolls, about Tom and the Troll, includes a nonsense word Tolkien made up: "boner". "Boner" at that time was a word meaning "error", although in said poem it seems to be just random rhyme-filler.
There are various subtler uses as well. Aside from writing seventy years ago, Tolkien was deliberately using a slightly archaic form of the language (at least when Men and Elves, rather than Hobbits, are talking.) For instance, when Aragorn says of the Nazgûl "they are terrible!", he doesn't mean they are of awful quality: he means they bring terror.
One way Aragorn demonstrates his kingly status is treating the war wounded after Pelennor: "The hands of the king are the hands of a healer. And so the rightful king could ever be known," as is said in Gondor.
Elrond is a skilled surgeon and ultimately the one who heals Frodo after his wounding at Weathertop.
Glorfindel may also have these, as his examination and working on the wound earlier is able to lessen Frodo's suffering.
Healing Herb: Athelas is used by Aragorn, once in a failed attempt to cure Frodo of his Morgul wound and later successfully on those affected by the Black Breath of the Witch-King. He's frustrated by the fact that it's become a folk cure used mainly by elderly men (or in other words, men his own age who don't have a Dunedain's lifespan) so that he has to hunt it down from them, or look for it on the roadside, while people are in need of it.
Brilliant line in the Jackson film: "Kingsfoil, aye, it's a weed." Everything you need to know about the way important things become diminished in memory over time, and about Aragorn himself, right there.
Healing Potion: Most of the 'learned' or magical cultures have something. The elves of Rivendell brew miruvor (which seems largely to restore from fatigue rather than mend wounds); the Ents brew Ent-draughts meant to act as food and aid regeneration (which have an unanticipated effect when used on hobbits or other mortal creatures, namely it makes them grow taller); the Numenoreans brought the herb athelas to Middle-Earth, though only the Dunedain still seem to know of its physical and spiritual healing qualities now. Even the Orcs have an ointment that seals wounds and a potion to renew strength, although the ointment is painful and leaves scars, while the potion is repellent in the extreme.
Helmets Are Hardly Heroic: Aragorn shows up to the Battle of the Pelennor Fields wearing a diadem (a royal heirloom of Arnor) instead of a helmet. Averted in an early draft, though, as originally he wore a proper helmet with a crown attached.
Happens briefly to Éomer during the Battle of the Pelennor Fields, when he learns that his sister Éowyn, who should have been safely home in Rohan, has died (or so he believes) in the battle. Followed by Unstoppable Rage.
"Then suddenly he beheld his sister Éowyn as she lay, and he knew her. He stood a moment as a man who is pierced in the midst of a cry by an arrow through the heart; and then his face went deathly white, and a cold fury rose in him, so that all speech failed him for a while."
Similarly, Sam shuts down for hours upon discovering that Frodo has seemingly been killed by Shelob.
A classical example with Aragorn, whose lineage is certifiably heroic going back about 70 generations, and originates from the three 'fathers' of both Elves and Men. On a minor note, Éowyn also argues that she's entitled to fight because of this trope.
Aragorn is also descended from Melian the Maia (an angelic being similar to a demigod) giving him Elvish, Human and Divine descent.
There are hints of adventures that the other members of the fellowship had before meeting the hobbits at Rivendell, such as Aragorn's capture of Gollum, or Gandalf's escape from the Ringwraiths. At one point Sam wonders if Gollum thinks he's the hero of his own story.
The Appendices mention entire other battles - notably one at the Lonely Mountain - that occur completely offstage in parallel with the events of the novels.
Glorfindel is a total badass who took down a Balrog all by himself. The event caused the Valar to make him their representative on Middle-Earth. So why isn't he first choice for the Fellowship? Because he's so famous, and feared, that the Fellowship would be anything but anonymous with him along. Apparently, Boromir, a renowned warrior of Gondor and heir to the title of Steward, Legolas, the Prince of the Elves of Mirkwood, and freakin' Gandalf are nothing in terms of fame and notoriety compared to Glorfindel. Unfortunately, Glorfindel himself doesn't get to do much within this actual story, and both film adaptations leave him out entirely.
Frodo and Sam already have a pretty close relationship before the quest (Elrond himself quipped it was impossible to separate them even by inviting Frodo to a secret council without inviting Sam), but it gets even closer as the quest progresses. Frodo has a lot of affection for Sam and tries to protect him even as Frodo himself grows weaker due to the Ring and his progressive injuries (even when what he's protecting him from is Sam's own tendency not to take care of himself) and Sam acts as Frodo's caretaker and bodyguard through Mordor. While some readers see all this as evidence for shipping, Tolkien said that the relationship is platonic, and Sam eventually gets married to his childhood sweetheart (with Frodo's hearty approval).
Legolas and Gimli turn into this over the course of the book, in the process overcoming Middle-Earth's long-lasting animosity between Dwarves and Elves. The appendices reveal that they took each other sightseeing through Middle-Earth, and finally sailed for Aman together on the last of the elven ships. In fact, Gimli is the only dwarf ever to make the journey to the Undying Lands, and he does it at least partially to be with Legolas.
Merry and Pippin are not seen without each other until they are separated by circumstance in The Two Towers, which is kinda a big deal for them (especially for Pippin, who learns in the process to be more responsible and less dependent on Merry).
Rivendell stands in a hidden valley in an otherwise desolate land, and though many stories mention it, few outsiders know of its location. The location of Lothlórien is well known, but its inhabitants isolate themselves to the deepest parts of the woods and refuse nearly all direct interaction with the outside world, causing their neighbours to regard them with superstitious terror.
The Shire, rather than any elf haven, lies closer to following this trope. Along with being generally unknown to most of the world, they also generally don't bother themselves whatsoever with outside business.
High Fantasy: One of the most notable examples of the genre.
Hiss Before Fleeing: Gríma does this, along with spitting before King Théoden's feet, before returning to Isengard.
Honour Before Reason: The Three Hunters, who are determined to reach Merry and Pippin after their capture even if it is simply to sit down and starve with them. The various people who spared Gollum's life also apply, since letting him loose was much more dangerous than outright killing him, but in the end this illogical act pays off, since Gollum is crucial in the saving of Middle-Earth.
Hope Is Scary: Éomer's reaction to learning that Éowyn was still alive.
Hopeless War: Sauron, having learned well from his many failures and defeats, is for all intents and purposes undefeatable by the time the Ring is found: his forces are virtually limitless, his base of operations is militarily impregnable, and Sauron was far too canny to overextend himself or attack before he was ready. It took Aragorn revealing himself and playing on Sauron's fears that someone might claim the Ring to challenge him to lure him into launching his attack early, and Gandalf notes after the Siege of Minas Tirith that the world of Men had barely survived what was effectively a strong probe, and they would inevitably lose through attrition.
Hope Spot: Return of the King gives one to Sauron of all people, along with a simultaneous Oh, Crap! moment. When Frodo puts on the Ring at the edge of the Crack of Doom, for the first time in millennia, Sauron knows exactly where the Ring is, and who has it. What's more, even though the Ring is now in the sole place where it can be unmade and Sauron finally realizes that was his enemies' plan all along, its current bearer has succumbed to the Ring's temptation in the absolute heart of his territory, right within his grasp, and stands absolutely no chance of contesting his will. Even though he was so terrified that he forgot about the war taking place on his doorstep, Sauron was also literally minutes away from returning to his full power if only Gollum hadn't been such a clumsy idiot.
Horsing Around: King Theoden is crushed under his own horse after it is shot by a Nazgul.
Hufflepuff House: Except for a brief mention where the three remaining ones are offered to Dain Ironfoot by Sauron if he betrays Bilbo's location, the seven dwarven Rings of Power don't factor into the plot at all, nor are any of their bearers seen or named outside supplementary materials. This is because, by the time of the story, they have either been destroyed or are back in Sauron's possession. In contrast, the three elven Rings are held by major characters (Gandalf, Galadriel, Elrond), and the nine Rings of Men are held by the Nazgul, the nemeses of the forces of good throughout much of the book.
Humans Are Average: Averted. Humans, instead of being average, are better known for their physical strength. Boromir is especially noted as being immensely strong. Dwarves, who usually get the "Strong guy" role, by comparison, are physically weaker, but have greater stamina.
Interesting twist on this, because the whole thing usually revolves around either their negative qualities or their mortality.
From the perspective of the Elves, humans are special in that they are permitted to die and depart from the world, while Elves are immortal and, when killed, continue on as spirits and can be reincarnated.
Hyde Plays Jekyll: Gollum playing Sméagol. For a while 'good Sméagol' manages to 'banish' 'evil Gollum'. After Gollum's 'return', he is the puppet master. He drives Sméagol on to continue acting as a good and loyal servant, putting up a front of being good Sméagol, but all the while actively planning his betrayal.
I Am Not Shazam: invokedThe "Lord of the Ring(s)" is Sauron. People get it wrong and are corrected in the book, as well as in real life.
I Am X, Son of Y: Modern-style family names are only used in the Shire and Bree-land. Everyone else works with patronymics.
I Call It "Vera": In spades which are probably also named. Justified in that the myths that the book is based on do this as well.
I Can Still Fight!: Éowyn tries this. They drag in Faramir to stop her; he points out that the army has gone too far and she can't catch up, and if the battle does come to them, she would be better able to fight if she rested now.
Gandalf, called Mithrandir by the Elves, Tharkûn by Dwarves, and Olórin in the West as an example.
This is in part by many characters having translations or versions of their names in the Common Language, Elvish, Dwarvish, or other languages. This also extends to many place names, some of which are even renamed after certain events.
Ignorant of the Call: Sam Gamgee refuses to accept the idea that he's any kind of hero. He only regards himself as 'here to help', even when he has to shoulder the burden of The Hero a couple of times.
I'm A Humanitarian: In the films, orcs will kill and eat one of their own if there's no other food. Averted in the novels - accusing somebody of cannibalism is fighting words! - but they rather enjoy eating humans.
I'm Cold... So Cold...: Frodo spends a lot of the time after he has been stabbed with the Morgul knife shivering and describing how cold he feels.
Immortality Field: Defied. The Numenoreans were blessed to live up to more than 300 years yet they still envied the Elves. Sauron tricked the Numenorean King Ar-Pharazon into thinking that those who enter and rule the Undying Lands will gain immortality (they won't; its native inhabitants are immortal to begin with and it doesn't matter where they are), but doing so violates the Ban of the Valar's restriction against mortals. For this transgression, Eru Iluvatar sank Ar-Pharazon's invasion fleet and Numenor beneath the oceans, removed the Undying Lands from mortal reach, and changed the shape of the world from a disc into a globe.
Immortality Inducer: The One Ring can produce a Blessed with Suck kind of immortality, as experienced by Gollum. It is also implied that any of the greater Rings of Power would have this effect on mortals, such as the nine held by the Nazgûl.
Improvised Armour: In "Durin's Folk", a chapter from Appendix A, dwarf Thorin Oakenshield gains his sobriquet after he uses an oak branch as a shield in battle when his own shield gets broken.
In Mysterious Ways: According to Tolkien's letters, the story is partially about Ilúvatar's guidance. It's only explicitly mentioned a few times, and none of God's names are ever even spoken; nevertheless, one could make a long list of the many, many suspiciously fortunate "coincidences" in the story — some very small, but all of which lead toward Sauron's downfall and the victory of the West.
Frodo is given the task of delivering the Ring to Mount Doom specifically because of this trait, but he actually turns out to be a deconstruction. Not only is the constant process of resisting the Ring's influence an enormous psychological and even physical burden on him, he eventually succumbs to its lure after lugging it all the way across Middle-earth to Mount Doom. No one, not even Frodo, is that pure.
Although he's not tested a tenth as much as Frodo is, Faramir shows greater nobility (or maybe savviness) than almost any other character by saying, "I do not wish to see it, nor know more of it than I do, lest I fall lower in the test than Frodo son of Drogo."
Samwise doesn't have to actually carry the terrible thing (ordinarily), but suffers every other privation Frodo does - to say nothing of watching his beloved friend dying on his feet. In addition to the Ring's own constant wiles, Sam has to fight his own, ordinarily very creditable desire to spare Frodo this agony.
Indian Summer: From the backstory, Aragorn's season with Arwen in beautiful, dreamlike Hidden Elf Village Lothlórien, at the end of which they get their Relationship Upgrade, and after which all the real hard work and war kicks in. It's the one time between the discovery of his lineage and destruction of Sauron that Aragorn allows himself some respite, and the summer is remembered very fondly afterwards.
Industrialized Evil: Isengard in particular, although Mordor shows some tendencies (and has a 4,000-year head start, particularly in the 'polluting the land' area).
Inertial Impalement: This is how Sam is able to seriously wound Shelob. The book explicitly states that he hadn't the strength to pierce her hide, but her attempt to crush him added her inertia to his strength.
Members of the Proudfoot family of Hobbits are quite insistent that the plural form of their surname is "Proudfeet".
Out-of-universe example: The Lord of the Rings is, according to Tolkien himself, not a trilogy. It is, in fact, six books plus appendices, sometimes published in three volumes.note In fact, Tolkien originally wanted the entire thing to be published as one book, but it was Divided for Publication due to paper shortages following World War II.
Intelligent Forest: Tom Bombadil's "Old Forest" seems to be something like this. The trees by themselves don't appear to be completely sentient (although Old Man Willow comes closest), but they all behave together to funnel intruders deeper into the forest.
Interclass Friendship: Frodo is the heir of one of the wealthiest hobbits in the area. Sam is the family gardener/manservant. While Sam never forgets that he isn't of Frodo's rank, their already good relationship becomes very close as the two make their way to Mt. Doom.
I Shall Taunt You: Aragorn does this to Sauron by using the Palantír of Orthanc. His audacity in showing and identifying himself - and his sheer will in tearing the palantir free of Sauron's control - convince Sauron that Aragorn is the 'new Ring-lord' he's been dreading ever since the One Ring resurfaced. Sauron immediately transfers the Nazgul from Ring-search to military duties, and sends his legions from their encampments in Gorgoroth toward the West, leaving Mount Doom virtually unguarded.
Frodo gets this to the most extent, in the end finally collapsing to the power of the Ring, and even afterwards never fully healing physically or mentally, and finding that even his home does not comfort him anymore.
Aragorn to some extent as well. He is destined to be the king of Gondor, meaning he has to ultimately relieve Denethor from power, who is none too keen of the idea, and causing conflict with Boromir, who feels that Denethor should be king instead of a mere steward and wishes to be king himself. Luckily, these two major sources of conflict are removed by Denethor and Boromir dying before Aragorn's return to the throne, and Faramir, who then inherits the role of steward, is perfectly fine with handing the realm back to Aragorn.
Gandalf gets some of this as well: he is chosen to help and guide the free people of Aragorn, but ends up always being the bearer of bad news, and the bringer of troubles, because he always goes where he is most needed. This earns him the nickname of Stormcrow by some. Gandalfs mission to install Aragorn as the new king of Gondor also sets much bad blood with Denethor, which causes Denethor to grow suspicious and paranoid, contributing to his eventual downfall into madness.
It Was a Gift: Quite a few, including that small, completely insignificant trinket Bilbo gave to Frodo. Gandalf emphasizes the fact that Bilbo giving away his "trinket" explicitly as a gift reduced the Ring's remaining grip on him. Compare what losing the same trinket did to Gollum.
Gandalf: Bilbo alone in history, as far as I am aware, has gone beyond talking about [willingly giving up the One Ring] and actually done it.