Keystone Army, in part: The destruction of the Ring kills Sauron, which confuses and thus incapacitates the parts of his armies which were more directly controlled by his will (e.g. the orcs), which makes them easy game. The generals of Sauron's human forces had various natural reactions, with some fighting on hopelessly while others surrendered or withdrew.
Kill It with Fire: The Ringwraiths, Shelob. Depending on how you want to look at it, the Balrog inverted the trope.
King Incognito: Aragorn for the first part of Fellowship is just Strider.
Knife Nut: Legolas has a long knife in addition to his bow.
Large and In Charge: Aragorn stood at 6'6'', which was considered very tall for Men at the end of the Third Age. His ancestor King Elendil was even taller at nearly 8'.
And the High King of the Sindar, Elu Thingol, was taller still!
Last Minute Hookup: Faramir and Éowyn get together rather abruptly at the end, having spent a week almost constantly in each other's company in the Houses of Healing.
On Eowyn's side, at least, it's not entirely unexpected: she's no longer duty-bound to care for her beloved uncle, has put her infatuation with Aragorn behind her, and has even received the recognition for heroism she always yearned for: in the midst of searching for a new purpose to life, she's in prolonged proximity to an admirable man wounded in the same campaign.
Last Stand: Or so everyone thinks at the Battle of the Black Gate.
Late-Arrival Spoiler: The Return Of The King is the title of the third book. Tolkien initially insisted on "The War Of The Ring" as a title to avoid the table of contents spoiling the (albeit secondary) story.
Although it was also pointed out to him that "The Return Of The King" did not necessarily imply "The Victory Of The King".
And Frodo's title for the story was even worse: The Downfall of the Lord of the Rings.
Since Frodo was writing contemporary history, most people reading it would already know the basics, such as that Sauron doesn't rule the world.
Layered Metropolis: Minas Tirith has seven levels, with a stone promontory jutting out from the topmost to overtake the rest of the city. It was built this way to be extremely defensible, with multiple lines of defense.
Leaning on the Fourth Wall: The chapter "The Uruk-Hai," which focuses on Pippin, includes a line by Merry that Pippin's actions in it were so impressive that there will likely be a whole chapter about them in Bilbo's book.
And then by Sam and Frodo in a strangely Genre Savvy conversation during the climb to Cirith Ungol:
Legendary Weapon: Narsil is the legendary sword that was forged in the First Age by a famous dwarven smith, wielded by Elendil til it broke, and then used by Isildur to cut off Sauron's ring finger at the end of the Second Age. It lay as a historical relic in Rivendell for three thousand years, then was reforged under the name Andúril at the end of the Third Age to fulfill one of the ancient prophecies.
Lemony Narrator: Mostly in the early chapters in the Shire and till Bree; again in the later chapters on the way back.
In the context of Tolkien's mythos, the Sun is not good for the elves, who see it as symbolic of the triumph of men over them (it is outright stated that the Sun symbolises the waning of the elves, and Galadriel implies in The Lord of the Rings that they see the dawn in the same way mankind sees the dusk - as symbolic of the end). The Elves frankly prefer the stars, which at least were already around when they were created.
Literary Agent Hypothesis: The author claims that The Lord of the Rings is translated from the Red Book of Westmarch, which was written by the hobbits (mainly Bilbo, Frodo, and Samwise).
It seems obvious in retrospect, given the changing nature of the narrator's voice from section to section. The story begins with Bilbo's homely descriptions of the hobbit characters' interaction, gradually changes to Frodo's scholarly and slightly purple narration throughout most of the rest of the book, and ending with Sam's down-to-earth, humble (but still educated) language towards the end — the second half of Book Six, detailing the Scouring and renewal of the Shire, is directly implied to have been written by Sam ("I have finished. The last few pages are for you"). As for the characters who didn't directly contribute to the writing of the Red Book, Merry has a remarkable eye for detail and consequence that strikes quickly to the heart of matters, and the narrator — probably Frodo — takes care to report his and Pippin's dialogue as faithfully as he can.
It's entirely possible that Pippin and Merry contributed to the Prologue, "Concerning Hobbits". Both of them seem to be quite swotted up on the history of their families, which are as close to nobility as anyone in the Shire can claim; Merry's interrupted spiel to Théoden on the subject of pipeweed is almost a verbatim copy of some of the prologue's remarks on the Leaf, and he is credited with an exhaustive treatise on the herblore of the Shire.
Load-Bearing Boss: Justified in a roundabout way. The fall of Barad-dûr coincides with Sauron's death, because it was built impossibly high by using the Ring to magically strengthen its foundations. Once the Ring was destroyed, the spells woven with it were shattered, and the Tower couldn't stand.
Losing the Team Spirit: This happens to the fellowship when Gandalf falls in Moria. Even though he's not really dead, they have no way of knowing this — Frodo and Sam don't even find out until after the quest is over! Aragorn manages to pull them together long enough to get them to safety.
Once Sauron realizes that Frodo has claimed the Ring for his own, he immediately shifts his attention away from the battle he is overseeing, abandoning his troops to their own devices. This leads to a sudden increase in Orc casualties, as they no longer have the Eye coordinating and motivating them.