Some people have critisized her for advancing the level of violence and serious issues in each of her successive Harry Potter books. I think what she has done has planned not only seven books that follow the adolescence of Harry Potter, but the developmental needs of growing readers. A young child, who is cognitively and socially developed enough to understand only so much about the world, can start the first in the series and 'grow' with it. As each book develops more challenging aspects to understanding it, the readers are stretched to grasp it.
Also, they are following Harry in age (because each book happens to be just perfect reading for a child the same age as Harry is in that volume), so their interests, fears, typical-for-the-age-problems they face are mirrored in Harry. This is a character who has become an invisible friend for many, many, kids.

In a way, she's revived a form of literature that was last seen (with enough quality to it to make it mainstream) with the Laura Ingalls Wilder books (the Little House on the Prairie series), which focused on the the growing lives of a famiy in prairieland, USA. Wilder's books were just as much historical fiction and a window into viewing what life was like for children and adults of that time period as they were a good story. All the books were based on her own history, and that of her family.
Rowling's books don't have any historical significance to them, they just tell a REALLY good story.
Let's not forget the immense value of really good storytelling!!

Rowling deserves recognition (which many have given her) for drawing out that part of us that demands good stories and defies whatever it has to in order to get them. Until Harry Potter and the Philosopher's Stone/Harry Potter and the Sorcerer's Stone was released, most children's and young adult books reflected an element of forced moralization or lesson teaching to them that was pushed by the powers that be, (our current social climate, the grand forces of a national education system, a nation with a war on violence -anyone note the oxymoron there?- and a political and religious system that deemed our youth were running wild). The end result was that publishers were pressured into publishing books that had some 'moral, educational or civic' value to them. What happened was the storytelling got lost and the 'issues' became more important.

If we look hard enough (if we look at all) we find many 'issues', 'morals', and 'lessons' to be learned in Rowling's books. But it's not the focus, it's a by-product of what happens when an author superbly crafts her characters, places them in real situations (real in the setting of the story) and masterfully tells the stories of what happens.

I think we could all, to paraphrase a quote from Mrs. Weasly, "Take a leaf out of her book"!