Here's a list of other items that can be included in that diversity label.
I live in the United States, so I do consider books that take place outside the United States part of that diversity label. And to go one step further, I like to read books that are set in locations other than the United States or Europe because Europe is almost as popular as the United States.
In most of the books I read, if religion is mentioned, the person is a Christian. But what about people who are Jewish or Muslim or even Hindu? I don't read about people who follow other religions very often so I would really consider that part of the diversity factor. And while we're at it, where are the books about atheists? And NOT the ones where they "suddenly" find their faith in God by the end. I am an atheist and I would love to read more books featuring them.
3. Medical condition or disability
I recently read a book featuring a character with muscular dystrophy. Do you know how excited I was to see that? I actually have a form of muscular dystrophy, so that was all kinds of awesome. I also recently read a book about conjoined twins and that was a first for me.
What does the term "diverse" mean to you? Are there any other factors that may lead you to classify a book as diverse?