Here’s what should be avoided in a cover letter:
typos
generic writing (address your cover letter to a named person and tailor your cover letter to the specific role)
writing more than one page
too small or large font sizes (stay between 10 and 12 points)
unprofessional fonts like Comic Sans or Papyrus (use a good cover letter font like Arial or Cambria instead)
negative comments about your current or previous employers