Diversity! They want to see what sets you apart and makes you unique from other applicants. Whether that be your job experience, extra curricular, or even frame of mind. Write about something that is important to you that will let your voice come through. Try to come up with something that is unique and interesting. These people read hundreds of essays so if you can get yours to stand apart from others and stick in the mind of the admissions officer, you will diversify yourself from the crowd and have a better chance at being chosen.