Ahhh a question that challenges me. I'm excited.
I'll just briefly premise this by saying i first read HP when I was 9. Harry Potter opened up the world of reading for me. I became obsessed with books because of Harry Potter. I hold Harry Potter responsible for the fact that I grew to love english - and then history - what I'm at university studying now. You can say that the books are "crap" but the indirect effect it's had on me and so many others is phenomenal. You can't deny or disregard that.
You're stating that the formula is threadbare and unoriginal. Which is fair enough in some ways - on the surface it does seem overdone. But you've got to think, there's a reason it's overdone. Kids love reading about things they're familiar with. As different as his life is, we can relate to Harry. He goes to school, has an enemy, has homework, has friends, has a favourite teacher and a least favourite teacher. It sounds like your normal kids day. But throw in magic and a evil villain who's out to kill you and you've got what most kids WISH their life was like. But that's explaining why it's so popular among kids, not its literary merits. From reading that it does sound very bland. But you're skipping over the "magic" (pun completely intended) of Harry Potter. The plot is fairly original, although I have no doubt you'll argue this. Not too many books mix fantasy and reality and even fewer in the present-time reality. The plot is very complex, particularly as the series progresses. You rarely find redundant details in a HP book. And the subplots alone could be made into a novel. They're captivating and intriguing. Feel free to dispute this, I'll take you on ;)
I'll address now you're saying that the maze to the philosophers stone was far too simple. I've heard this argued amongst Harry Potter fans before and the answer inevitably leads to Dumbledore. Somehow Harry managed to find everything he needed to know in order to get to the stone. And apparently under the blind eye of Dumbledore. I've heard many a theory that Dumbledore intentionally lead Harry to the stone in order to test him or mature him. And you may say that's sick for an old man to risk a boys life, and I agree. I'm completely undecided of Dumbledore's "manipulation" throughout the series.
You're saying the beasts are unoriginal. Have you read the entire series? Here's a list of creatures in the HP world http://harrypotter.wikia.com/wiki/Fantastic_Beasts_and_Where_to_Find_Them_(fictional)
As you can see the list is fairly large. Each one of those creatures has a classification and a description of its abilities in JKR's world. Many many of them are entirely original. Many are hybrids of those in mythology. I don't think that's a bad thing - I think it adds depth to the series and just shows the work JKR put in when writing it. She obviously put in a fair bit of research. The fact that you're saying Smeyer is more creative than Rowling just made me vomit everywhere. Sparkly vampires? Smeyer is constantly criticized because her vampires are completely unlike "vampires" and it's argued because they diverge so completely they're not even vampires. Maybe that's not a bad thing. Maybe that's showing her spectacular 'creative flair'. Or maybe it's just, to paraphrase Smeyer "it's my world I can do what i like". JKR respects mythology which is clearly shown in her books. It's another link between the real world and Harry's. There's also her use of names - for example Argus Filch, Alastor Moody etc. I think it adds depth to the series and if anything is a show to her creativity.
The complexity of the world JKR has created should also be mentioned. It's the reason that people like me can obsess so completely. There is SO much. We have a complete history of the wizarding world. That alone is complex and interesting. We have goblin rebellions, wizarding wars, witch burnings etc. etc. There's the history of Quidditch, the teams of quidditch, the 700 rules of quidditch. We have the magic of different countries, the jobs within the magical world, magical currency, means of transport, wand lore, blood lines, I could go on forever. Basically the wizarding world is COMPLEX. JKR has created a different world that we can immerse ourselves in a second.
The characters are fantastic as well. Now I'm sure you'll disagree but I find them so relatable. They have real flaws and there's not a Mary Sue in sight (maybe Twilight should take a leaf out of HP's book). My favourite character personally is Remus Lupin - a man who is so intrinsically good but also flawed. That's the thing with HP there are so many characters who seem so minor, but have such rich personalities and stories. People's favourite characters range from Ron and Hermione to Snape to Dumbledore to Hagrid to Dobby to Neville. The characters are 3D. Look at Snape. Such a complicated past, yet I don't think that justifies his actions. He's such a multifaceted character. The fact that his love perseveres through the years is a testament to his true personality I think and an example for all.
The themes and morals in the series are exemplary as well. Despite all the adversity he's forced to overcome, Harry remains a good person. The main theme of the series is love versus hate. Harry's ability to love saved him from death. It's a perfect example for the kids who are reading it and continues to inspire me to be a better person. And I'm not wanking on here. Seriously.
Now my favourite part of the series - I've been saving this 'til last. The foreshadowing! The 'gleam of triumph' in Dumbledore's eyes in Goblet of Fire is just a foreshadow of what happens right at the end of Deathly Hallows. And even simpler ones. I'm currently reading Azkaban and when Lupin confronts a boggart he see's a 'silvery orb' - we don't discover to the end of the book that it's a full moon. Sirius Black is mentioned in the first chapter of the first book but doesn't come in again 'til the eponymous third book. That's what kept the fans speculating. What was the significance of Harry's eyes? Will Harry get with Hermione or Ginny? Was Dumbledore's "please" a beg for Snape to kill him or a beg for life? You cannot deny that the books are an amazing example of mystery.
I'm an adult (I just scrape in there) and i still love Harry Potter. The characters are rich and developed, the plot intriguing, the world complex. I love the foreshadowing and the obvious research JKR has put into the books. Just my 2 cents