How much does the subject matter of the essay count? Is it better to write about how you started an organization to help orphans in China over baby-sitting your cousins?

While the more impact you've had on the world the better, an essay about a spectacular achievement won't always trump more humble experiences. How you write is more important than what you write. That pertains to more than just writing style—the admissions committee will be reading between the lines to draw conclusions about what kind of person you are. It's a good bet that someone who's accomplished enough to start a charity is an amazing and generous person, but if a high-achieving do-gooder comes across as arrogant and self-satisfied, an essay about one's cousins that portrays the writer as interesting and thoughtful could be seen as more successful.