I think that the books that are loved forever are books that have either had a hand in revolutionizing some aspect of writing (such as Tolkein's Lord of the Rings with the modern fantasy genre) or have touched a wide variety of people. Most classics have very universal themes, and in some cases, have had specific roles in history- like Hurston's Their Eyes Were Watching God.
I don't think Rowling will be gone in 150 years. I think Harry Potter will become a children's classic series. It's done almost all of the above and is not, contrary to popular belief, merely "fun reading". It's some serious stuff. I doubt schools will ever teach it to their kids, but they don't teach The Golden Compass, either, and I don't think that that one's going anywhere. Like Narnia.
But Meyer will disappear because her books are poorly written and though they've touched hundreds of people across the world, they're the "phase" types of books. People get into them for a while and then they grow out of them because there's not much left to love after 1) you've grown out of high school chick flicks and/or 2) you realize how poorly written it is.
I hope that helped answer the question.
