USA Today has a nice article on points to consider when preparing a will. The most salient is that beneficiary designations over-ride will provisions. If a parent's will divides all assets equally between 3 children, but an account is owned jointly with one child, that child will inherit the entire account negating the parent's wishes.
Personally, the other key point is that people are discouraged from preparing their own wills. My children's school seconds that advice.