Great discussion so far. Hypatience, PHLSTUDENT22, monochrome spring and others are making solid points.
I'll just add that the writing sample is probably the most important part of your application. Four things about that:
Write something you know a lot about. (If you don't know a lot about any one topic yet, get a short reading list from a professor.)
Try to keep your main idea simple. Your first couple of paragraphs should summarize it clearly—so clearly that even people working in other areas will know what you're trying to say.
You won't win points for sounding dismissive, but you will for sounding fair-minded.
Please, please, please: simplify your prose. Avoid clutter, delete fancy words, use short sentences, and stay on topic.
Hope that helps. Good luck, future folks.