On the flip side, if you loved your job, your coworkers, your boss, the cappuccino machine, everything about the place, don't write page upon page waxing rhapsodic about all that you're giving up by leaving. A brief, heartfelt compliment to the company, or a thank you, is fine.
A resignation letter can also help your boss and your company start the transition process, get a replacement for you approved, and start planning internally, etc. The paper trail is beneficial all around. So what should go into your letter? Let's look at some examples of what to do (and perhaps more importantly, what not to do).