I recently reread Harry Potter, which was awesome and amazing and lots of fun, and it got me thinking about why I like the books so much. There's obviously something about them that I really like, since I've read these books at least 10 times, but before this reread I've never really thought about why.

So, what makes Harry Potter good?

Characters
The first thing and probably the most immediately obvious thing is the characters. The characters in the series are phenomenal. There are so many of them that are so well fleshed out that you have to relate to at least one, especially because a lot of them follow basic archetypes that you've probably seen in real life: the bitchy teacher Umbridge, the grumpy and scary teacher Snape, the firm but fair teacher McGonagall, the bookworm Hermione, the eccentric weirdo Luna, the paranoid old veteran Moody, etc. These simple character also become more layered and complex as the series progresses, and over the course of the books you really feel like you get to know them and you start to genuinely care about them.

Because of how strongly you connect with each character, every death in Harry Potter cuts extremely deep. Every time I reread and relive Dobby apparating to Shell Cottage with a knife in his chest or Fred getting killed by a curse is genuinely upsetting, and it's a mark of a good author and a good book to make you really feel and lurch when characters die.

But sadness is only a very small part of the emotional range you feel with the characters. Neville going from a small chubby boy getting bullied by his grandma and Snape in Potions to defying Lord Voldemort, pulling Griffindor's freaking SWORD out of the burning Sorting Hat, and decapitating a giant snake housing a fragment of Voldemort's SOUL in Book 7 is an immensely satisfying moment of strange pride in a fictional character, almost as if you're celebrating the growth of a close friend.

You also hate Snape, from Book 1 to Book 5, and then feel weirdly vindicated in Book 6 when it turns out he's been a spy all along and kills Dumbledore, and then angry when he cuts off George's ear, and then, in an insane twist in Book 7, turns out to be a TRIPLE agent who actually has remained in love with Lily for years and has actually always been loyal to Dumbledore. The reader's relationship with Snape is a crazy rollercoaster, and if you're telling me you weren't surprised by that then you're either lying or a lot smarter than I am.

The most important part though is that for those of us who started the series as kids and ended them as young adults, we grew up with these characters. I remember Ron and Harry becoming friends on the train, Ron, Harry, and Hermione becoming friends after beating up a troll, Hermione getting petrified, Hermione and Harry saving Buckbeak and Sirius, Ron and Hermione arguing over Krum, Ron sucking at Keeper, Hermione organizing the DA, the three of them sneaking into Gringotts, their kids going to Hogwarts, 19 years later... I read these books in my most formative years, and experiencing the series in my youth made the characters feel like friends that I've had for decades.

As a side note, making readers despise characters is also a wonderful authorial skill, and it's objectively impossible to read Book 5 without feeling a deep, deep loathing of Umbridge and associating hem hem with all authoritative bitchiness.

Story
Harry Potter is a pretty long series. It goes on for seven books, each of which is pretty long (especially towards the end) and a ton of shit happens, but it manages to remain consistently entertaining and creative. This is a very tough thing to do, as often writers get confused by their own story threads (GRRM, where is book 6), or just run out of material to write about (Bleach), but each HP book has a pretty different and interesting story, and no matter what type of story you like, there's something in HP that you'll probably enjoy.

If you're into fantasy, HP is a very large and intricate universe of magic, yet with enough normal, human elements to make it believable and immersive. That is the fantasy dream.
If you're into mystery, most of the books in the series has some sort of mystery and some sort of big twist (who is trying to get into the locked door on the 3rd floor? who killed the Potters? who opened the Chamber of Secrets? who put Harry's name in the Triwizard Tournament?). I can't really remember what I was thinking when I first read book 4 but I'm sure finding out Professor Moody was actually a Polyjuice potion'd up dark wizard probably fucked 4th grade Justin up a lot.
If you're into love stories, there's plenty of those in HP: Ron and Hermione, Harry and Ginny, Tonks and Lupin, Bill and Fleur, Snape and Lily (doesn't technically count but I'm counting it anyways and I'm willing to fight you over it).
If you're into intense thrillers, parts of the story are crazy hype, like Harry finding out Quirrell has Voldemort stuck on the back of his head, the Triwizard cup secretly being a Portkey, or when Dumbledore comes to the ministry in Book 5 (announced by Neville shouting DUBBLEDOOR, which I still find tremendously funny).
If you're into a classic "good triumphs over evil" story all of Harry Potter is basically that. There are some sad parts but HP is fundamentally a very happy series.

Universe/ Background
HP takes place in a very immersive and expansive universe. There are a bunch of spells, a bunch of cool magical creatures (nifflers, hippogriphs, dragons, thestrals...), a bunch of history, a sport, tons of cool places (Hogsmeade, The Burrow, and especially Hogwarts). People I talk to about HP still remember spells, some people literally play Quidditch as a real sport with brooms between their legs, and I still hear people describe beautiful, awe inspiring places as being "like Hogwarts!!!"

Theme
The theme in Harry Potter is not mind bogglingly confusing or difficult to understand and remains pretty consistent from the first book to the last, but that doesn't mean that these simple themes are not important. Loyalty and love and friendship and solidarity and courage are ideas that resonate with children and adults alike, and it doesn't matter who you are or how old you are, everyone can enjoy and appreciate Dobby's loyalty, Harry, Ron, and Hermione's friendship, Neville and Luna's courage, and Harry's loving sacrifice for his friends.

Obviously the series is not perfect, and people love to point out all the random plot holes in Harry Potter (I especially hate Time Turners), but what all these things boil down to is that Harry Potter is relatable. As kids, confused and unsure of ourselves, J.K. Rowling built a universe for us where we felt like we were home, we had friends, and we were taught that love, loyalty, and friendship are the most important things. This is something that so strongly resonated with so many of us that even now, people are still taking "Which Hogwarts house are you in" quizzes, people still brag about the amount of HP trivia they know (try mispronouncing Wingardium Leviosa with your friends, guarantee at least one person will correct you immediately), and people still remember the bitter disappointment we all felt when a Hogwarts owl didn't come before we turned 13. That's good stuff.