You must look at your follow up letter as a marketing/sales opportunity. This may be your last chance to influence the interviewer and convince him to bring you back for a second interview or even extend a job offer.

After expressing your thanks for the interviewer's time and interest, concisely reiterate why you want the job, why you think you are the best choice for the job, what you can give to the company that other applicants can not and any other selling points for this particular position.