One of the characteristics of a good cover letter is that it does not only summarize the contents of the resume but gives the hiring manager a reason why you should be considered for the position. It should show a personal interest about why you would want to work for the company. The cover letter should also briefly discuss the applicant's work experience. Bear in mind that it should be concise enough to give the reader an idea of what the applicant used to do for a living without really giving all the details. A good cover letter should also be interesting to read and should be free from any misspellings or grammar errors. You can find more tips on what makes a good cover letter at http://www.askamanager.org/2007/06/what-does-good-cover-letter-look-like_13.html.