Erica Swallow gives you the bullet point breakdown of all the fundamentals of a worthwhile job description in an article at Mashable. She breaks it down into essential details and formatting tips. For the essentials, you want an accurate job title and summary, five to ten key responsibilities (that begin with a present-tense verb), both mandatory and preferred qualifications, a specified job department and supervisor, an overview of the company, and of course recruiter contact information. As for formatting, use bullet points whenever possible, get explicit and concise with the language used, and generally try to attach a tone to the job description that fits the overall tone of the company.
For more tips, you can read the full article here: http://mashable.com/2011/06/03/how-to-write-job-description/