Step back and put yourself in the place of the people who will read the letter. They want to know what you can do for the company, not what you hope to accomplish for yourself.
Spelling and grammar should be flawless, of course. But other mistakes are easy to make: spelling a person's name wrong, using an incorrect job title for a person, and confusing a company's products with those of another company. Consider any mistake to be fatal.
If it looks like a form letter, it may be ignored. If it has something unique to say, it may get read. Mass mailings may indicate to a hiring manager that you are not really interested in that organization.
People who appear desperate or demanding are not taken seriously. Employers appreciate enthusiasm, but not too much.
When you are submitting your letter with postal mail, don't forget to sign your name! You would be surprised at how often people forget. Don't get clever with your computer and printer: sign it neatly in blue or black ink.