Not every resignation letter you'd like to write is a good idea. Here are some examples where it's best to leave things unsaid.
Ill-Advised Resignation Letters
There are some ill-advised resignation letters that have been requested for this site, and examples are provided for the best, most tactful way of framing the subject. However, the best resignation letters leave out things that could be litigious, hurtful, or damaging. Here are some that you might reconsider:
Leaving due to stress/sickness/injury: You're establishing yourself in writing as a person who might be considered sickly or flaky, even if that isn't true. If you recover and seek your old job again (or a new one), the employer might think you don't have what it takes.
Alternate Suggestion: Say you're leaving "for personal reasons."
Leaving due to vacation policy/PTO/overtime: Take it up with HR or mention it in your exit interview. If it's in your resignation letter, it's going to look petty, nitpicky, or overblown.
Alternate Suggestion: Just leave it out.
Leaving due to subpar payment/products/environment: Again, leave negative statements off the written record. This person may have to write you a recommendation or reference someday.
Alternate Suggestion: If you're leaving for a better company, try to frame a positive comparison, e.g. "I'm sorry to leave this company. I hope you understand that I simply couldn't pass up the opportunity for an increase in salary."