If you can afford to lose out on most, if not all, job opportunities, sure, go ahead and make demands -- everything from the salary and benefits to vacation time and how often you'll be allowed to wear jeans. But no one likes demands, and they're totally inappropriate for a cover letter. You should save any discussion of specifics for the interview stage -- if you make it that far -- and even then, make sure you're actually discussing rather than demanding.
Unfortunately, sometimes well-intentioned job-seekers don't realize they're being demanding. One common cover letter mistake is including wording about how you're looking for a unique opportunity where you can be adequately challenged. You may think such language shows you're no slacker, and are eager for difficult tasks, but employers often look at it as if you're expecting them to find you a position and specific projects that fulfill your career goals, and not those of the company [source: Work Source].