Difficulty in Dexterity-Based Platform Games
March, 2004 - Victor Nicollet
This article explains a little about game design choices for setting the difficulty in platform games. Good read.
Improving Player Choices
March, 2004 - Tracy Fullerton, Christopher Swain, Steven Hoffman
This articles discusses game design issues in regards to how it is important to give the player choices and how to do it right.
Game accessibility guidelines
This site offers lots of tips on how to design your game with accessibility in mind.