I read Sorcerer's Stone yesterday, after I had read Deathly Hallows and Half Blood Prince last week.
The last 2 books in the series compared to the first book is like comparing Filet Mignon to hamburger steak. Not that the first book is necessarily bad, but it's certainly a children's book with a very simple story and writing style. The first book has 309 pages, the last book has 759.
I don't think JK Rowling ever expected the series to become such a cultural phenomenon. I think she originally intended to write a series of simple children's books in the model of the first book. The first book is like the Hardy Boys/Nancy Drew with magic. I think that was her original intent. As the series got more popular, she made the story vastly more complex, giving us an epic series instead of a string of magical kiddie mysteries.
One comparison would be "The Hobbit" and "The Lord of the Rings". The Hobbit is much more of a fairy tale. The Lord of the Rings was an epic.
Unfortunately I think that may make it difficult for new readers to get into the series. Young readers will find the later books too challenging, while older readers will find the early books too simplistic.