The most important traits to emphasize in your essay the technical and interpersonal skills that will help you to succeed as a doctor. When answering why you want to be a doctor or explaining your skills and qualifications be sure to demonstrate the medical knowledge you learned in undergraduate school. Technical knowledge is a large factor in getting into medical school. However, also be sure to give examples of ways you interacted with patients when volunteering or doing hospital work. Just having the knowledge is not enough, you must show an admissions committee that you have the ability to put into action what you have learned.
Other traits that an admissions committee will want to glean from your essay are maturity, honesty, motivation, enthusiasm, empathy, humor, compassion and commitment. A successful essay will describe a competent and well-rounded applicant. But don't just claim to possess positive traits --- cite examples from your life.
Stay away from stories about your hard luck, unless that hard luck truly made an impact in your commitment to medicine. Focus not on the obstacles themselves, but on the way you overcame them. Also, steer clear of controversial issues and your positions on them. This essay is not about your ethical convictions, it is about you personally. Similarly, avoid overly technical writing. It won't help the reader relate to you.
Be sure you are answering the question that was asked and not straying off topic. Write naturally and concisely. Don't rely on overused words like "good" and "nice," but don't waste too much time with a thesaurus either. Finally, double-check your grammar, punctuation and spelling. An excellent essay can be brought down by sloppy writing.