CollegeRecruiter.com has a great post on topics to avoid during job interviews.  According to Steven Rothberg, you should avoid topics such as sex, politics, and religion unless they are directly relevant to the position (they almost never are) as well as any discussion of compensation, your old boss, your significant other, or your health.

In general, you should be very careful about how much personal information you divulge in an interview.  There is never a good reason to bad-mouth your previous employer, even if everything you're saying is true.  While he is listening to your tale of woe, your prospective employer is thinking that in a year, you will be sitting in front of another interviewer whining about his company. 

Also, don’t allow yourself to be swayed by an interviewer that seems compassionate.  Remember that the interviewer is not your friend and her first priority is to be loyal to her company.  If you are asked why you left a job, answer with a neutral statement like “the commute had me in the car four hours a day” or “I couldn’t pass up the opportunity to work in the X field.” 

Keep in mind that your principal goal in an interview is to project an image of professionalism, maturity, and competence, and so you should avoid saying anything that might portray you in the wrong light, and, as Rothberg says, aim to ask the right questions and give the right answers!