As part of a series of posts on novel-writing, this week on Naked Authors, James O. Born discusses dialogue. He relates the common author experience of hearing from readers who object to profanity, asking in conclusion, "How do you feel about reading uncomfortable language in a book? Does it sweep you up in the realism of the story? Or does it turn you off?"
A character's language only makes me uncomfortable when it doesn't fit my impression of the character from the rest of the story (or from previous books in a series). For example, Robert B. Parker's P.I. Spenser has a reputation for poetic repartee, so when I notice him using cliches or old jokes, it bothers me.
On the other hand, it baffles me that readers would pick up books about unsavory characters yet expect them to refrain from dirty deeds and profanity. If readers object to profanity, violence, or whatever, they can choose books about polite, repressed pacifists instead.