Since the essay is the first impression, it will structure the light and viewpoint they take when analyzing your reference letters and your interview. In that light, if there's a really great accomplishment that your reference will include on his/her letter (ie. you did a volunteer mission, did some traveling, or in fact, had any really deep and meaningful experience), consider "pre-introducing" that into your autobiography and mention how it affected your life or influenced your thinking.
When the admissions committee sees this mentioned again from your referees' point of view, it will really cement that experience into the admissions committee's mind.
Bearing that in mind, my opinion on the content of the essay is to mention anything that won't be reflected on your transcript that you would like to talk about in the interview. This includes scholarship awards, meaningful volunteering experience that you actually want to talk about, traveling, and past experiences that may have cemented your desire to study medicine. If there are any huge gaps in your transcript, ie not minor, you would likely want to explain in your autobiography that, for instance, the reason you took a year and a half off from school, was to get a job to become financially solvent. Or that the reason you didn't take a full courseload was that you were sick, or taking care of another sick person, and couldn't do well in all the courses so you decided to do well in a lesser number of courses. (My English teacher would kill me for that above sentence.)
Your interviewers will most likely ask you several questions on the material that you supply them. Be sure to save copies of your autobiographies and refer to them before you go for interviews. You'll want to come up with positive stories about everything mentioned in your autobiography. Also expect to have to explain what you've learned from each of these memories and lessons; med schools tend to eat stuff up regarding personal development and maturation as those candidates tend to have put significant thought behind their desire to enter medicine. It goes without saying that you should have thought long and hard about this decision before you apply.