Not only did Deus Ex include the feel of a branching story line, it also makes an attempt at changing the way your character acts and reacts based on previous actions. Everything from how many people you kill to whether or not you wander around in the women’s bathroom have an affect on other characters’dialog, and yours as well. The game achieved a sense of agency unmatched in any game previous and (in my opinion) every game since, including its own sequel. Just the fact that the plot never felt forced and that there were so many options available to the player throughout the game makes it certainly worth recognition as a quantum leap in game driven storytelling.
Deus Ex made the biggest quantum leap in story telling because it gave the player freedom to choose the method of playing through the story-line, whether the method is violent, sneaky, or skillful.
Vitas Povilaitis, Graceful Boot