Common elements that people see in books they like to ban

There is a difference between challenging and banning the books.Books can be challenged if parents,libraries,school board attempt to have material removed.A ban means they succeeded.
Books have been banned all over the world for many reasons.Usually the intention is to protect people,especially children from controversial ideas or opinions, and it is usually something to do with sex,violence,religion,racial views and profanity.Books are often forbidden from reasons of ideology,religious reasons in societies that do not promote democracy and freedom of speech.There is sometimes strong lobbying ,powerful and influential ,that can persuade government to put a ban on a certain book.In some drastic cases authors are threaten by death for the ideas they promoted in their book and they are against religious views of their government.Like Salman Rushdie for his book The Satanic Verses .Sometimes books are threatening to stimulate people to think different way,to rebel against government ,and governments and politicians like to stay in power and in control of the information and education system and the books that are part of the curriculum.
Sometimes is the Church that warn about some books like Da Vinci Code,heavily criticized by Roman Catholic Church and called shameful and unfounded.