Say things in a simple, straightforward way, and don't rely on a thesaurus. As with your resume, use action words to create dynamic sentences. Be specific and get to the point. Your cover letter must be intriguing enough to get the reader to look at the resume, but should be only an introduction to the resume, not a repeat of it. It's difficult to sell yourself as unique if your letter reads like every other one in the pile.
Nobody wants to hire somebody with an attitude.
Above all, don't sound like you're begging for a job. A hiring manager may wonder why you're so desperate. Be confident, but not arrogant . Don't be negative or too humble. Tell them you're qualified for the job, but don't demand it.