The limits of individual will
Frodo cannot destroy the Ring by choice. No amount of courage or determination is enough. Tolkien builds the entire quest around this idea — the Ring can't be resisted, only destroyed by accident and grace. Students writing about willpower or fate should start here.
The cost of victory
Winning the war doesn't restore the world. The Elves leave. Frodo is too damaged to stay. The Shire is scarred. Tolkien refuses to let victory feel clean, and that refusal is one of the book's most honest moves.
Kingship earned through service
Aragorn's right to rule isn't just hereditary. He heals the sick in the Houses of Healing, leads a suicide mission to the Black Gate, and commands through example. The book argues that legitimate authority comes from what you do for others.
The overlooked as decisive
The people who turn the war are a hobbit, a shieldmaiden, and a creature everyone wanted dead. Frodo is too small to be a soldier. Éowyn is barred from battle. Gollum is pitied when others wanted him killed. Tolkien keeps giving the decisive role to the ones the powerful dismissed.
Endings and the passage of time
The book is saturated with endings. The age of Elves closes. Old friendships scatter. The world the characters fought for moves on without them. Tolkien treats this not as tragedy but as the natural order — things end so new things can begin.