I talked about what drew me to my field, and what experience and talents that I have that I can apply to what I'll be learning and my career - in my case experience with digital media and an interest to pursue those developments. Google statement of purpose, it sounds silly but it helps quite a lot...
...especially telling you how NOT to do it. I keep in mind for anything I apply to that these people will be going through hundreds of the same thing, so making it enjoyable and interesting while explaining a facet of your personality is important. Also kind of hard, but very worth it to show you're putting major effort into the application.
Also, I think you can upload your statement of purpose if it doesn't fit in the box. Mine was 1200 words, or so and I uploaded it as a .doc for all of my applications.