Here are some things to check for when you're making sure your document is clear and sounds professional:

The writing is clear and easy to understand.

A person who's hiring new employees only has a short amount of time to read each cover letter, so it's important to make sure the writing is clear.  

The sentences in your letter shouldn't be too long or too difficult to understand.  

The sections of a cover letter should be in the correct order: first the heading, then the introduction, then the body, and then the conclusion.

The writing is specific.  

Writing the sentence "I have good computer skills." is not as clear as writing "I can type 55 wpm and am comfortable using Gmail."

Using specific examples of ways you've used and developed your skills in the past helps a cover letter stand out to potential employers.

The writing sounds professional.

Make sure there isn't slang in your writing. Avoid using words like awesome, cool, or bogus.

It's also important to sound respectful in your writing. "I want an interview." sounds less professional than "I'd like to interview for the position."

Telling the person who's hiring for the position what you can do for the company instead of asking them what the company can do for you is a good way to make sure your letter sounds professional.