The second part of R. Scott Campbell's writeup on the creation of Fallout covers the original setting ideas, the core design principles based on their pen and paper fandom, and the eventual conflict with the GURPS license.
Another core decision that stemmed from our Role-Playing addiction was the idea of creating your own characters. Sure, most RPGs allow you to allocate attribute points and choose a male or female body we wanted you to be able to create a character that allows you to play the way you want to play.
How do you want to play the game? Do you want to be the gun-wielding tough-guy? The buff melee brute? Maybe the stealthy assassin? The nimble guy who can't be hit? Or maybe the guy who can talk anyone into anything? All of these choices (and any combination between them) must all be valid. By simply choosing from a few skills and abilities, you can tell us how you want to interact with the game.
This also meant that however a player specialized their character, they still had to be able to get through the game. Initially, I underestimated all the permutations that this decision actually meant. What if you had a character that was really good at persuasion, but not trained in combat? If a player wanted that kind of play experience, we had to deliver. Thus, those (Charisma-boy) characters can easily gain allies that fight for them, and are able to talk their way out of most situations in the game.
Rule #4: Let the player play how he wants to play.