I thought of a very wonderful quote. I'm not sure where it came from. I might have read it somewhere before. Maybe it is many quotes brought together. Maybe it is entirely of my own creation. I don't know. Anyways, I find it to be very beautiful and thought-provoking...

"Twilight is a children's book poorly disguised as great literature. Harry Potter is great literature poorly disguised as a children's book."

I doubt I wrote it. I'm not capable of coming up with something so witty and beautiful as that.

Allow me to explain what it means. It means that Twilight makes people think it is great literature because of its content. Most of it is about "romance" and has many mature themes, like sex. That doesn't mean that Bella and Edward are constantly talking about it or doing it, but it is always on their minds.
But even though the content itself seems more grown-up than the charming school of Hogwarts, with its cute little wizards and witches learning basic magic, making friends, and growing, Twilight is really a children's book. It is written so that a young child could understand it (though I hope young children don't read it!) with very basic sentence structure and basic vocabulary, with the exception of big words awkwardly thrown in to random places. It tells the story in the most simple way possible. It doesn't elaborate where it is supposed to and has unnecessary and distracting details where it shouldn't.
As for Harry Potter, it is beautifullly written. It has a main plot (Harry's quest to destroy Voldemort), but also many subplots that make the story interesting wihtout making it confusing. The characters are well-developed and the descriptions are stunning without being repetitive or tacking five adjectives onto one noun. Many assume it is a children's book because the story is about witches and wizards. And they are partially correct: kids love Harry Potter because of that. However, that doesn't change the fact that everything was carefully planned, researched, and written wonderfully. Adults love Harry Potter because of its subtle complexity and wonderful plot, action, suspense, romance, love, emotion, sense of loss, self-conflict, and excellent story. These are the things that make Harry Potter truly great literature, no matter where the story takes place or whether or not they are wizards.

"Twilight is a children's book poorly disguised as great literature. Harry Potter is great literature poorly disguised as a children's book."

I must have read it somewhere, but I truly wish I could claim this quote as my own.