1. "I meet the requirements for the position." Hundreds of candidates will meet the requirements for the position. That's not enough to get you a second look. You want to explain why you're an excellent candidate, not just an adequate one.
2. "I'm hard-working and a great communicator." And probably a team player and an independent self-starter, too. These are cliches that cause hiring managers' eyes to glaze over. Worse, they don't convey anything of substance—the fact that you've assessed yourself as these things will hold no weight whatsoever with employers, who prefer to assess these things for themselves.
3. "I'm a visionary leader." But not very humble, apparently. If you're truly a visionary leader—or a master communicator, or whatever other brag you're tempted to make—it should be evident from the accomplishments you've listed on your resume. Proclaiming this about yourself comes across as, well, weird.
4. "You won't find a candidate better qualified than me." Unless you've seen the rest of the candidate pool, you have no way of knowing that. This comes off as needlessly cocky hyperbole—and it's generally inaccurate, to boot. If you're really stunningly qualified, the hiring manager should be able to spot that on her own. Simply proclaiming it, especially when you have no basis to know if it's true, doesn't reflect well on you.
5. "Dear sir or madam." In most industries, this will come across as an antiquated, stuffy salutation. If you know the hiring manager's name, use that instead, but if not, simply writing "dear hiring manager" is fine, and won't make you appear as if you come from an earlier century.
6. "I'll call you in a week to schedule an interview." Job seekers can't unilaterally decide to schedule an interview, and inappropriately pushy to pretend otherwise. Some people believe that asserting themselves like this will demonstrate persistence and good salesmanship, but in reality, it just annoys hiring managers.
7. "I'm willing to work for below the salary you're offering." Candidates who write this generally are hoping that it will get them an interview when their qualifications alone wouldn't have. But it doesn't work, because hiring managers want to hire the best person for the job, have budgeted a certain amount for the position, and aren't going to take a weaker candidate just because she offers to work for less than the budgeted salary.
8. "I've attached my college transcripts, a list of references, a 15-page writing sample, and my last performance review." Unless the job posting specifically asked for these items, don't include them. At this stage, employers just want a resume and a cover letter. Don't overwhelm them with items they haven't asked for and might not want. Wait until you've progressed further in the process, and then ASK if they'd like these items.
9. "Please contact me if you'd like to see my resume." Job seekers occasionally send a letter of interest in a job without including a resume, to the great mystification of hiring managers everywhere. If you're writing to a company about potential work, you must include your resume. It's the first thing an employer will want to see, and they have no way of knowing if you're someone they'd like to speak further with without first seeing that.
10. "I really need a job. I'm desperate." Hiring managers might feel sympathy for you if you're desperate, but that's not going to make them hire you. Your cover letter needs to focus on why you'd excel at the job you're applying for, not how badly you need it.
Alison Green writes the popular Ask a Manager blog, where she dispenses advice on career, job search, and management issues. She's also the co-author of Managing to Change the World: The Nonprofit Manager's Guide to Getting Results, and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development.