Say something like:
a) Why you wanted to do that course, as darkliight mentioned above
b) The reputation of that uni--blab on about their research reputation, good academic environment with top academics, excellent facilities in which to facilitate your learning. If the uni's known for a particular area of research or specialises in offering certain units of study (in generic degrees) that you're looking to make a career in, include that as well. So on that last point, something like, x uni offers a very strong environmental law unit which I'm interested in, as opposed to y uni which either offers that but is not popular, or doesn't even offer it at all, within the law degree.
c) Just on that point about how great x uni is, probably a comment or two about the excellent non-academic opportunities the uni provides, and probably something like, if you're doing Science there's a Science students society or whatever, that can provide support and bring like-minded people together, and if not, say that it's known for its vibrant lifestyle and you'll get to meet other people and broaden your horizons and this could reflect on your work--new ideas, or whatever.
That should be enough for 500 words, I think--that's only half a page. Good luck!
