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.
You'll have plenty of time to vent personally about your work grievances as you leave, but really, you stand to gain nothing by putting negativity into your resignation letter. And unless you're leaving the state and your current industry, you always run the risk of running into the same people later on. So you're far better off taking the high road and making sure your exit is a classy one.