Writing a good cover letter takes a fair degree of practice, skill, and knowing what prospective employers want. After a couple years of writing them, most job candidates are very good at hitting all the necessary points—they don’t include generic salutations, always end the last paragraph with a call to action, and never, ever forget to proofread before sending in.
Where even experienced cover-letter writers sometimes run into trouble is tone. Applying to a storied institution that’s been around for a century—think IBM or a Wall Street bank—often demands a more formal style of writing than, say, a bright young startup; in the latter case, you can probably get away with dropping a few colloquial phrases into your narrative without the HR manager dinging you for excessive casualness. That being said, your communications should be professional in every circumstance, as well as direct.
Which brings us to the last point: Some candidates make the mistake of devoting a significant portion of their cover letter to praising their prospective employer. While this seems like a great idea on the surface, it’s actually a waste of the HR manager’s time. The cover letter is a vehicle for demonstrating why your skills and experience can help a company grow, and you only have a very limited amount of space in which to make that point. Save the praise (and the reasons why you think the company’s worthy of it) for the actual job interview.