As a couple pointed, it felt a little sloppy towards the end, I guess understandably so since if I remember correctly, she wasn't planning on writing 7 books. However, in many ways how "pop music" many other types of "better" music aside and takes all the limelight, I felt the Harry Potter series was kind of the same thing. The plot wasn't really original, I mean come on just a kid who finds out he has great and ends up being the hero, big deal honestly. Sure, some interesting twists and plots, but I felt I've read many more enjoayble stories and series in my life.
The movies? They were adequate, again, IMO nothing special, only truly popular because all those kiddies (and adults) were able to see their favorite character portrayed as a real person.
But yeah, as the OP said, the story is quite simplistic in nature and the writing is really not that special, pretty basic vocabularly and such. It just happened to catch fire. Certainly much credit is due to JKR but like most forms of art (music, writing, painting, etc) you obviously need that luck to get the success and again it still doesn't necessarily make it "best." Just like in music, even if much of today's pop music world is spewing out songs that are breaking record views and sales, doesn't mean they will be hummed and played 300 years later like great classical music.
Just a kid? More like a kid who makes friends in a magical world while learning from some wise guy and trying to get free from his uncle's abuse while having romantic relationships and journeying all over England trying to fight off the greatest dark wizard of all time.
Honestly, there's like 1000 other complex characters.
I honestly think Harry Potter will be remembered as a classic. It is written with a cleverness and wit that appeals to all ages, not just teens, and anyone can get absorbed into that world. I mean, just name one mentally sane person who doesn't want to go to Hogwarts. If it doesn't become a classic...well, it was better than fluff which are only remarkable because the author lived a long time ago.