In the immortal words of Yogi Berra, “It ain’t over till it’s over.” This was learned the hard way by a job seeker I recently heard about. It illustrates the need to address every detail and to take nothing for granted.
When talking with a recruiter today, he told me about a candidate whom he presented to his client, the hiring company. The candidate had a solid résumé, interviewed well, and followed up with a letter after the meeting. That is where it all went wrong.
The thank you letter was poor. It was filled with awkward language, grammatical errors and other defects. The contrast between the well written résumé and the substandard thank you letter was stark. It was so bad, the client lost interest and declined to go further with the candidate. The letter cost him the job.
I will state the obvious: The thank you letter is important, but it has to be great. If you don’t write well, get someone else to help. Tend to the details and know that the job isn’t yours until you report for the first day.
Here is a related story to tell you what you must include.
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Here is another piece on the importance of thank you notes.
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Bill Florin is the President of Resu-mazing Services Company. He can help you with a great cover letter.
Work: Getting It & Keeping It 
I find it troubling that a seemingy good candidate can’t write a simple cover letter.
Agreed! The incident definitely left its mark on the recruiter. I was meeting with a group and he made his point well to everyone that this was an incredible flame-out.
The thank-you letter is important. Sometimes when torn between two candidates, the only one who writes the thank-you may get position.
You’re right, and as this experience teaches, it has to be good. I always encourage my clients to recap a point or two from the interview and how s/he will help the company using skills and experiences related to those discussion points. Thanks for your comment!