• You make it boring and formulaic -- especially the first paragraph.
• You make it too long or with extremely long paragraphs.
• You ask for an entry-level job.
• You include an unsolicited salary request.
• You include negative information.
• You use a sexist salutation.
• You rehash your resume.
• You sound too desperate and willing to do anything.
• You display ignorance about the company to which you're writing.

Follow this link for some examples.