If you are wondering how to answer this question during an interview for a new job, it's best to focus on the positive. Saying that you are after new challenges is way better than saying you couldn't stand your old boss! During an interview, your prospective new boss could be sitting across from you and he can only wonder how you'll feel about him!
Even if thinking about your old job makes you feel sick to your stomach, put a proactive spin on things. Don't say, "I left because I didn't like 'X, Y and Z." Rather, emphasize what you do like about the new job and what you are looking forward to. Speaking highly of the new company (but don't overdo it) will not only impress your prospective employers but will take the focus off of whatever problems you might have had in your old job.
Even if thinking about your old job makes you feel sick to your stomach, put a proactive spin on things. Don't say, "I left because I didn't like 'X, Y and Z." Rather, emphasize what you do like about the new job and what you are looking forward to. Speaking highly of the new company (but don't overdo it) will not only impress your prospective employers but will take the focus off of whatever problems you might have had in your old job.