The sample supplemental essay for Duke is typical of what an admissions office frequently encounters. At first glance, the essay may seem just fine. The grammar and mechanics are solid, and the writer clearly wants to expand his or her education and become a well-rounded person.
But think about what the prompt is actually asking: "discuss why you consider Duke a good match for you. Is there something in particular at Duke that attracts you?"
The assignment here is not to describe why you want to go to college. The admissions office is asking you to explain why you want to go to Duke. A good response, then, must discuss specific aspects of Duke that appeal to the applicant. The sample essay fails to do so.
Think about what the student says about Duke: the school will "educate the student in a variety of subjects" and present a "range of challenges and opportunities." The applicant wants a "broad education that spans a variety of areas." The student wants to be "well-rounded" and to "grow."
These are all worthwhile goals, but they don't say anything that is unique to Duke. Any comprehensive university offers a variety of subjects and helps students to grow.
As you write your supplemental essay, take what I call the "global replace test." If you can take your essay and substitute the name of one school for another, then you have failed to address the essay prompt adequately. Here, for example, we could replace "Duke's Trinity College" with "the University of Maryland" or "Stanford" or "Ohio State." Nothing in the essay is actually about Duke.
In short, the essay is filled with vague, generic language. The author demonstrates no specific knowledge of Duke and no clear desire actually to attend Duke. The student who wrote this supplemental essay probably hurt his or her application more than helped it.