I just read that cover letters are often just scanned and the more "key words" you use that fit with the job description and job duties, the better. Just don't misspell or used weird or colored stationary (other than, you know, white, off white, neutral), use a classic font.
It also depends a lot on the industry and size of company - if I were applying to be a scientific climate modeler, I would expand on my abilities; if I were applying for something for which I was overqualified, I would definitely address my willingness to jump in for more mundane tasks.