Not just humor, but the overall tone of your application essay is remarkably important. It's also difficult to get right. When you are asked to write about your accomplishments, those 750 words on how great you are can make you sound like a braggart. Be careful to balance your pride in your achievements with humility and generosity towards others. You also want to avoid sounding like a whiner -- use your essay to show off your skills, not to explain the injustices that lead to your low math score or failure to graduate #1 in your class.
Personal essay is one of the richest, most vibrant and rage literary forms. As it gives you an opportunity to exceed the limits of the standard essay, to utilize conversational tone, dialogues, settings and characterizations. Before writing your work you should try to plunge into you inner world, get to know yourself from the inside and gain the understanding of the self. Writing your personal essay intends to vent your thoughts, ideas, interests to the others and lets you drive toward candor, self-disclosure, honesty with yourself. After such a revelation with yourself you'll be surprised by the quantity of positive sides you've got.