Books that are banned contain themes that "adults" tend to see not fit for the younger generation
to read:
Extra marital affiars
Suicide
Magic
Anti-Religion
Questioning of Sexuality
The lack of Adults in a child character's life
The defiance of parents
Excessive death
War
Use of alcohol/drugs by parents,children, or supporting characters
The personification of animals
The use of the imagination
Actions society considers taboo