Fantasy stories generally lift from the same mythological source bank. Actually, if you want to get super scholarly and esoteric about it, every story (especially the more sword-and-sorcery themed variety) is the same basic "hero's journey" archetype detailed by Joseph Campbell. But fantasy stories -- King Arthur, Lord of the Rings, The Chronicles of Narnia, Willow, Peter Pan, Conan the Barbarian -- build their world out of the same objects. You have magicians, dragons, unicorns, gnomes, and elves popping up to do battle, and they've made their way from medieval tapestries and texts to our modern movie screens.
Many writers, such as the brilliant J.R.R. Tolkien, were scholars of language and literature in their own right and used original sources to craft their world. Middle-earth is a wonderful blend of Finnish, Norse, and Anglo-Saxon mythology, though it's most famous inhabitants came to life in the back of an exam book. One day, Tolkien scribbled "In a hole in the ground there lived a hobbit," and an entire saga was born after he decided to find out what a hobbit was.
Harry Potter came to life in a similar manner as J.K. Rowling gazed out of a train window, and imagined a lonely boy speeding off to a new world. Rowling crafted a mythology that's nearly as complex as Tolkien's. (A glance through Harry Potter's Wikipedia reveals family lineages, diseases, and battles not even discussed in the seven books.) But Rowling was a canny crafter of worlds, and her wizards borrow from Arthurian legend, Greek mythology, medieval history, and notably from Tolkien himself. Some might call it uncreative theft (and there are certainly a few eye-rolling grabs), but Rowling did it in such a way that Potter is simply the latest in a tradition that includes Merlin and Gandalf. It's one big mythology rather than an independent one.
For those who have either never read Harry Potter and Lord of the Rings, or lack the silly elephantine memory to categorize the similarities, I've taken up the task. I've ignored obvious and common stuff -- such as Harry and Frodo enjoying a similar "hero's journey," wizards with beards and pointy hats, fights with dragons and trolls -- and focused on particular themes, objects, and scenes. You'll either come away with a deeper appreciation of both stories, or dismayed at their parallels. You may also laugh at me for being obsessive enough to rattle it off, but hey, this is the stuff that wins trivia contests.
1. A Long Time Ago, in a Country Called England
As any Tolkien nerd will tell you, Middle-earth is our Earth, but in a prehistoric form. J.K. Rowling's series is equally archaic, as the events of Harry Potter actually take place in the 1990s, and the final battle of Harry Potter and the Deathly Hallows takes place in 1998. While Harry's readers see him as a boyish and heroic contemporary, the eerie truth is that he's already old enough to be their father. Didn't you always wonder why pre-wizard Harry lacked so much 21st-century technology?
2. No Sexy Librarians Among Wizards or Witches
I imagine wizards are like any scholar one encounters. Their brains are always working overtime, and they don't have time to organize or file away knowledge for future generations. Both the One Ring and the Deathly Hallows vanish from common knowledge (along with that pesky Chamber of Secrets!), and become objects only dusty kooks fuss over. They are so obscure that they are believed to simply be legend with no basis in fact. That's a bit problematic when the evil overlords desiring them (Sauron, Voldemort) decide they would help with that "enslaving the world" business.
3. Sauron and Lord Voldemort Started Out Ordinary
To those reading The Lord of the Rings, Sauron is an immense and faceless force of evil. His reach spans centuries and he reigns over an entire region that's been stripped of life. But nerds who pour over The Silmarillion know that Sauron is a pale imitation of his original commander Morgoth. Sauron was (to use very basic terms) a wizard just like Gandalf, but he became corrupted and emboldened by the evil of his superior, Morgoth. Likewise, Voldemort started out as an ordinary half-blood wizard named Tom Riddle. While a lot of things went into the making of Voldemort -- mainly a lonely childhood -- one could argue it's the dark influence of Salazar Slytherin (of whom Voldemort is a descendant) that nudges him on his way.
4. Gandalf and Dumbledore are brilliant and yet weirdly ineffective
Don't send me hate mail! I studied loads of mythology and literature, and I know the mentor's task is to eventually cut the apron strings and send their protégé into battle. I also know that neither Gandalf nor Dumbledore expected to take fatal plunges off high places. But as a reader sympathizing strongly with Frodo and Harry, you can't help but wonder why neither wizard gave them a little more instruction or support while they had extra time. A mysterious rune scrawled in a book seems decidedly useless when you're a hungry kid on the run from Death Eaters. And a "We won't be taking the pass of Cirith Ungol" back when they were packing at Rivendell would have helped Frodo out immeasurably.
5. The Magical Bling Bling
The One Ring is infused with Sauron's power, and leeches a corrupting influence to anyone who wears it, wields it, or is simply around it. The Horcruxes have a similar effect, as evidenced by Tom Riddle's diary (it possesses Ginny) and the Slytherin locket. The locket wears so badly on Harry, Ron, and Hermione that they take turns wearing it and succumbing to its awful influence.
6. Spiders. Why Does It Always Have to Be Spiders?
Tolkien had a terrible fear of spiders, which is why The Hobbit and The Two Towers both feature some of the most disgusting and vile arachnids to attack on page or screen. Clearly, they survived primeval Middle-earth by way of Hogwarts' Haunted Forest where they feast on unsuspecting visitors. And surely you noticed the King of the Spiders was named Aragog, which I like to think is a spider's corruption of Aragorn or possibly Hagrid's mangling of the name.
7. A Little Fuzzy on the Good / Bad Thing, Here
It's human (and hobbit) nature that we refuse to believe that things are really all that awful. Oh come on, Voldemort is dead. Please, the One Ring was lost. Quit your complaining, and get back to your day job. But then an evil eye appears over Mordor, or the Dark Mark over England, and you realize the crazy old man and his short sidekick were right all along.
8. An Elegant Weapon for a More Civilized Time
Legendary blades have been a part of mythology since we first learned how to forge metal, but they actually become more prolific as civilization gets better at making them. A hero like Aragorn needed an Excalibur-like blade, and so Tolkien gives him Anduril, the Flame of the West. By contrast, the wizarding world relies on wands and incantations for their battles, yet only the sword of Godric Gryffindor can defeat basilisks and Horcruxes.
9. Hooded Fiends of Horror
An evil overlord is only as scary as the minions he sends after you. Sauron's henchmen are his nine Ringwraiths. Their screech can wake the dead (metaphorically speaking), and their blades can corrupt your soul and turn you into one of them. Voldemort happily deploys very similar fiends called the Dementors. They are mostly silent and inspire misery from within, but they will suck out your soul given half a chance.
10. The Water-Walking Dead
Everyone loves zombies and ghosts, and it's not surprising to see armies of both pop up in Tolkien and Rowling (the Army of the Dead, Voldemort's Inferi). But Harry and Frodo have remarkably similar encounters with festering and fishy corpses. Tolkien's Dead Marshes (lifted from the battlefields of WWI) are more seductive and ethereal, luring Frodo into the water with eerie whispers and lights. Voldemort uses Inferi in a number of places, but we only encounter them in a drippy seaside cave, where they attack the careless.
11. Shed My Light Over Dark Evil
Frodo is given very little to go into Mordor with, but one of his most vital and beautiful defenses is the Phial of Galadriel. A mere bottle of water from Galadriel's fountain, it held the light of Earendil to be "a light in dark places, when all other lights go out." It proves to be just that for Frodo and Sam, and just touching it eases the pain of carrying the One Ring. Dumbledore's Deluminator seems pretty humble in comparison -- it initially just seems like a cigarette lighter that can suck up and store light -- but it also seems to store a more ethereal light. It leads Ron back to Hermione as she speaks his name, suggesting that it too is meant to be a similarly powerful solace.
12. If You Are What You Eat, Then I Only Want to Eat the Good Stuff
For a land universally (and unfairly, I might add) mocked for its cooking, it's funny that both The Lord of the Rings and Harry Potter feature some of the finest foods cooked on paper. Bilbo's teas in The Hobbit can really only be described as "sublime," and the fledgling Fellowship enjoy some pretty wonderful meals thanks to Tom Bombadil and elvendom. Even lembas bread sounds delicious! Harry Potter's food is even more fantastic. Butterbeer, pumpkin pasties, every flavor beans, the hearty meals cooked by house elves and Mrs. Weasley are enough to drive a reader to distraction.
13. They Ain't Making More of the Stuff
A key part of most mythology is that we were preceded by some kind of supreme beings. In Lord of the Rings, it's the elves who have all the knowledge of the ages. Everything elven (weapons, armor, food, houses, poetry) or dwarfish is automatically superior to anything the humans or hobbits can create. Harry Potter has a similar conceit, but Rowling takes a decidedly unglamorous approach, as the best stuff (the sword of Godric Griffyndor, for instance) is made by goblins.