Obsidian's Five Hard Lessons Of RPG Design on Gamasutra recaps points made on the topic of RPG design from a talk given by J.E. Sawyer of Obsidian Entertainment. He offers a detailed breakdown of the following five principles:

Mechanical chaos is frustrating, what you can perceive is the most important thing, strategic failures are the biggest disappointing failures for players, the idea of player vs. character is a false dichotomy, and good gameplay is better than whatever your ideas or whatever the player's expectations are.