I often become invested in stories. While I am reading, I become part of that story. It's no secret that I've always struggled with my weight, so when the protagonist makes cracks about another character being a "fatso," etc. I get completely jerked out of the story. In my head, I think "He/She would make fun of me, too," and it makes it difficult for me to enjoy the story.
It also bothers me when the "good guy" calls someone retarded. I read a book recently in which the protagonist was bothered by a jerk. Now, this annoying secondary character was truly a jerk, but it seemed to me that the main character should have been above making comments about his weight and even joking to another character that the guy was "special ed."
Now, I'm intelligent enough to go with the thought that these types of action are sometimes just characterization and are devices used to lead the character to the lesson that needs to be learned. However, when I read more than one unrelated book by an author, and these actions are present in each and every book without the character learning from his mistakes, I assume that these incidents of cruelty and/or insensitivity go beyond characterization and are giving us glimpses of the author's own personality. When that happens, it becomes more and more difficult for me to read books by that author. I've given up completely on certain authors because of this. Why would I continue with an author whose books are just going to make me feel bad about myself or angry on behalf of others?