I have written and edited many cover letters, and I am happy to share my experience.
A cover letter is an instrument by the author to convince the reader that the author is a suitable candidate and to create further interest in the reader in the attached documents if any. Cover Letter is a formal document. It accompanied by a resume is a representative of the author's personality. The reader forms an impression about the candidate by reading first the cover letter than the resume.
Cover letter should primarily address 3 questions - What, why and how?
What : What is that the authour wants? This could be a position in the company, or a seat in the academic institution etc.
Why: Why should the reader give the position to the author. There could be other competitors, why should the author be chosen? Here the author should explain his strengths.
How: How is the author going to add value to the organization? This is a critical section, most often ignored. If the author can explain how will be an asset and not a liability to the organization, it will give an edge to the application and create an interest in the mind of the reader.
Use of abbreviations should be avoided. If used full form should also be mentioned. Proper grammar and punctuation speaks of the discipline in the candidate. Space in the cover letter should be used wisely. More important things should occupy more space. Irrelevant information can put negative impression in the mind of the reader.
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