Just about every day I have students who ask me to look at their resumes and cover letters. I usually tell them two things about their cover letters: 1) Your cover letter is boring, 2) It talks too much about you. This hurts and puzzles them. Then I explain.
"Look," I say, "you're a great person, but no one really wants to hear about you. If you go down to the local pub and start telling some nice-looking lady or guy how wonderful you are, they'll either pour a beer on your head or run away screaming. But, if you tell them how wonderful they are, watch their eyes light up. Watch them buy you a beer!"
Obviously if you approach someone without the right panache and sincerity, they might still dump a Miller Lite on your cranium. No one wants to be flattered. But they (we) never tire of hearing sincere compliments about ourselves. In the case of a cover letter (and a resume), the recipient wants to know how what you are saying will help them fill some need of theirs, not yours (as in, I need a job!).
So, assuming you've first got the reader's attention (by the way, you can grab their attention in a cover letter by: 1. asking a question, 2. citing a great quote, or 3. telling a brief and interesting story), the next thing you do is tell them how you will fill their needs.
For example, you may begin your cover letter by asking a question, "What does Microsoft want in new hires?" After you ask the question (and have their attention), you give the answer (they be waiting for it). "Microsoft wants dedicated, enthusiastic, energetic, educated and self-motivated engineers." Then, you quote your previous boss, Joe Bagadonuts, who said, "Billy Bob was the most energetic and enthusiastic young guy ever to come to us from CMU." The rest of the letter shows how you understand the needs of Microsoft and highlights the many successes you have had in your young life producing just the kind of outcomes Microsoft requires. In other words, you write about them!
I tell my students that nothing they write is about them - cover letters, resumes, memos, e-mails, reports, RFP - none of it, not even a love letter (especially not a love letter). Anyone who receives a written message of any kind asks 1. why did you write to me, 2. what do you want me to do, 3. why should I care? In other words, they're saying, "Tell me quickly how this message benefits me because I got 230 e-mail messages today, 72 IMs, 43 tweets, 61 telephone calls, 11 letters and a partridge in a pear tree. I'm too busy to screw around."
This is doubly true for someone who is receiving 322 letters and resumes for one job opening. If your letter is one among the multitudes, you need to grab the reader's attention and differentiate yourself , which you can do by writing an interesting cover letter and showing the reader that you understand his/her/the company's needs.
The writing is not about the writer. Who is it about? The Audience. Right, always the audience! Or, as Peter Drucker said, "It is the recipient who communicates. The so-called communicator does not communicate. He utters. Unless there is someone who hears, there is only noise." And, they ain't gonna hear about you if all you talk about is yourself.
Don't add to the noise. Know the reader's needs. Connect with the reader. When you write your cover letter, put yourself inside the reader. Write from the reader's point of view. Remember, whatever you write, "It ain't about you!"