In 1997, based on a search in OCLC WorldCat, there were 83 English language books written about magic that would be considered Juvenile fiction (search term - Su: Magic Juvenile Fiction).
One of those books was Harry Potter and the Philosopher's Stone, published in the UK. The US market would get the book in 1998 (retitled with Sorcerer's Stone), published among 90 such fictional works.
This book would go on to sell 300,000 copies by 1999 in the UK and roughly 11 million (hardback AND paperback) in the US; top bestsellers lists; spawn a series of equally mass-selling books - nay spawn an entire industry of toys, foods, and other books; get made into blockbuster movies; and basically turn childrens' literary publishing into a financial behemoth as hundreds of other would-be Rowlings scribbled out pale imitations to cash in (with only a handful - Lemony Snicket comes to mind - showing any originality deserving our love).
The deal is, why? Why this book? What was it about the first Harry Potter book that made it so popular?
The reasons why are that firstly Rowling created memorable characters and memorable settings, and secondly like any good author Rowling stole (c'mon. Every artist a cannibal, every poet a thief...) from the good and/or great storytellers that came before.
The earliest comparisons Rowling got were to another well-known children's author, Roald Dahl, whose classics (Matilda, James and the Giant Peach, Charlie and the Chocolate Factory) were detailed child's-eye-view-of-the-world tales replete with adult (and child) grotesques. Are there any true differences between Matilda's uncaring parents the Wormwoods and Harry's Aunt Petunia and Uncle Vernon Dursley? Draco Malfoy and Veruca Salt are practically siblings (hate to think of them as boyfriend/girlfriend shudder. Oh God I hope there's no fanfic on that). Dolores Umbridge and Mrs. Trunchbull have to fight it out as Worst Teacher Ever. These characters are archetypes: bad parents, spoiled children, officious and uncaring bureaucrats, and all of them bullies to boot. Note too the detail of names, how they roll off the tongue as literally defining each character and yet made colorfully off-beat.
Rowling also borrowed heavily on a form of literature better recognized in England than in the US: that of the Boarding School coming-of-age tales. Such tales were prevalent when boarding schools dotted the British landscape, mainly the Victorian and Edwardian periods. Post-WWII era with a decline in such schools being replaced by regular public-funded schools also meant a decline in such stories, replaced by standard coming-of-age tales. Boarding school stories focused on basic themes: friendships made, bullies defeated, sporting events won. Said sporting events usually cricket or football (no broom-riding sports I'm afraid, old bean).
It might surprise, but Rowling also steals from a British author more popular than her: Agatha Christie. Potter books are - for the most part - plotted like murder mysteries: Whodunnit? (also How'd They Do It? and Why'd They Do It?) Who's stealing the Philosopher's Stone? Who opened the Chamber of Secrets? Why after 13 years is Sirius Black coming after Harry Potter now (or is he...?), and how is he doing it? Who really betrayed Harry's parents to Voldemort? Who put Harry's name in the Goblet of Fire? What's Voldemort want with the Department of Mysteries? Who is the Half-Blood Prince? What are Horcruxes? And the most important question of all: When will Josh and Donna make out? Oh wait wrong fanfic obsession... And that was resolved halfway through Season 7 anywho, but I digress.
Last but not least, the real reason Harry Potter sparked so much interest: Rowling followed the basic rules and plotlines of the Monomyth. And while Joseph Campbell's critics can argue that over-reliance of the monomyth as simple plot structure has led to a lot of crappy fiction, you can't argue when it's done right (Star Wars, the Matrix, even Star Trek). And Rowling gets it right. It also doesn't hurt that she follows a lot of fantasy-themed elements set down in The Encyclopedia of Fantasy, a reference that was in the works just as she published the first Potter book.
She created foremost a world that is both mundane but magical. Obviously, she created a Hero (Harry Potter) but also loyal Companions (Ron Weasley and Hermione Granger) who provide the Hero with vital tools: Ron gives Harry access to the Weasley family who act as advocates and guides to a Wizarding world he's unfamiliar with (as well as providing a surrogate home the Burrows) while allowing Ron opportunities to shine separately from his successful older brothers; Hermione provides knowledge and problem-solving skills that otherwise would lay dormant. There are additional Companions as well, notable among them Hagrid, Neville Longbottom and Luna Lovegood. Harry has his Mentor in Albus Dumbledore, although Remus Lupin and later Sirius Black provide Mentoring as well. And there's the Pariah Elite formed as the Order of the Phoenix, the wizard resistance formed to fight Voldemort and his Death Eater minions. And Snape... I'm tempted to place him as a Threshold Guardian, but arguably as the most complex character Rowling created pinning Snape down to any one Role is tricky. One more obviously is that Rowling created a Dark Lord (literally) in Tom Marvolo Riddle AKA Lord Voldemort, who threatened both Wizarding and Muggle worlds (The Land) with Plight... and like all Dark Lords he may be powerful but can be maddeningly blind to obvious flaws in his various plans to rule/destroy the world. Voldemort violates Rule 6 nearly every book he appears. Sheesh. Oddly enough, Voldemort never evens tries to violate "Rule 34. I will not turn into a snake. It never helps."
With the pieces in place, she follows most if not all of the monomyth rules faithfully: the Call to Adventure comes with that first letter from Hogwarts (addressed to the abused Harry's "Cupboard Under the Stairs"). The Refusal of the Call isn't Harry's fault: his aunt and uncle despise Wizardy and hoped by abusing Harry they would crush it out of him. Supernatural Aid comes via Hagrid who ensures the Call to Adventure is heeded, and Hagrid is the first to escort Harry into the Wizarding World (Diagon Alley) and help him locate tools (Wand, Owl, School Supplies) to assist in his survival. The Crossing the First Threshold and the beginning of the Road of Trials are basically Harry's first journey to his new school Hogwarts, during which he meets his Companions (Ron, Hermione and Neville) as well as Enemy/obstacle Draco Malfoy who acts as a bully and minor threat for most of the overall series. Draco is merely a representative of the True Threat Voldemort, reflecting Voldemort's bigotry and sadism but unable to truly achieve the malice of his Dark Lord. Past this, the book series is replete with Thresholds to pass and overcome, and various Belly of the Whales that Harry must survive: Entering the final chamber to confront the person stealing the Sorceror's Stone; Entering the Chamber of Secrets to save Ginny; Entering the Shrieking Shack to save Ron and confront Sirius Black (and the Truth and Atonement with the Father that Sirius represents); Entering a labyrinth that would lead to Voldemort and the Death Eaters at the end of the Third Trial of the Triwizard Tournament; Entering the Department of Mysteries to rescue Sirius Black but instead facing a trap; Entering Tom Riddle's childhood cave to recover a Horcrux; and with the last novel Entering an otherwise impenetrable goblin bank to steal a Horcrux, Entering the Room of Requirement to locate another Horcrux, and Entering the Forbidden Forest to finally confront his own Death (?) at Voldemort's hand. That's a lotta whale bellies.
There's Apotheosis with Harry's self-sacrifice at Voldemort's hand in the final book, but that leads to the Boon, the wisdom and knowledge gained in the Afterlife. With the Boon (knowledge that Harry is no longer a Horcrux, and the realization that Voldemort doesn't have true mastery of the Elder Wand) Harry considers for a moment the Refusal to Return, then acknowledges he must Return to bestow the Boon to everyone else by destroying Voldemort once and for all. He returns to the mundane world but is Master of Two Worlds (the World of Magic and the World of the Afterlife), and exults in his Freedom to Live by showing he was and remains unafraid of Death, unlike Voldemort who fears Death and is destroyed by his own Killing curse.
There's probably a more detailed way (probably a whole book) of fitting the Harry Potter series into the Monomyth structure (I didn't even touch on the Goddess stuff), but the point is Potter-as-Monomyth works. And despite all the complaints about the Monomyth, it DOES create easily-read, easily-recognizable tales that appeal across the broad spectrum of storytelling (well, at least within Western/Eurocentric Literature). That's why Harry Potter got to be so popular 10 years ago, and why it's still popular today, and why it may yet remain popular 100 years from now.