While I'm taking Information Storage and Retrieval this semester, I am also taking Children and Young Adult Literature. While in that class I have gotten the opportunity to read some really great books. A lot of my favorites happen to be on banned book lists. For example, I recently read Toni Morrison's The Bluest Eye and the anonymous memoir Go Ask Alice. Both books deal with identity, sexuality, violence, and various other confrontational issues that might seem alarming and can make one uncomfortable. But still, no matter how uncomfortable one becomes or disagrees with something in a book, it is still unconstitutional to restrict or ban those books.
It seems to me that a lot of the issues that initially cause a book to be "put on trial" are books that deal with very real issues that shouldn't be suppressed, but explored. Such as issues involving sexuality, ethnicity, or even basic insecurities that a youth is going to have to eventually confront. I've included a graph that I found on the American Library Association website that ranks the different reasons that a book is challenged. You might be surprised when evaluating what you personally might consider a threat to our youth.