To your point about how to word your reasons for leaving, you could just meet with HR (if they have HR) to verbally have an "exit" interview. But, I think it depends on where you go next in your job. Would you need some of your current employees or bosses to write or give recommendations for you? This might determine what you will say. Some people prefer not to say much about reasons for leaving so that they won't burn any bridges.
Sadly, many people are often not truthful in exit interviews since they are worried about potential backlash. This means the company does not get the information it really needs to make improvements.
Russell is the vice dean and the director of the Executive Coaching and Leadership Development Program at the University of Maryland's Robert H. Smith School of Business. She is a licensed industrial and organizational psychologist and has more than 25 years of experience coaching executives and consulting on leadership and career management.