I used to think they were looking for cultural diversity, of which I have none. So I wrote to suit my own uniqueness in regards to the majority of applicants - e.g. a phd, work in a cancer hospital and that I'll be 29 when class starts.
So, you have to brainstorm - What makes you unique? What separates you from pre-med automatons? Are you nontraditional - i.e. older (which = more life experience on an essay) Anything interesting happen to you that you may not have covered? If you have, go into more detail. Any graduate work? Have you done any research or worked in a lab for an extended period of time? Any publications? Start up any clubs? Were you a non-science major? Do you play the autoharp? Do you have leadership experience? Care for a sick individual? Can you do hand farts? Are you an oustanding chef? Write about anything that will set you apart from the rest of the class and manipulate it into sounding like the best thing in the world.
Andrew