Games and Stories
The parallels between pen-and-paper RPGs and video games don't end with their simulations, however. Both are games that tell stories, or perhaps more accurately, offer stories that have gameplay. A novel concept, if you take a step back.
Today most gamers take it for granted that games have stories, yet most traditional games -- chess and checkers, tag and hide-and-seek, baseball and basketball -- don't have any story.
They don't have plot, characters, or themes; in fact they barely represent anything at all. Games pose abstract problems or challenges that players are tasked with solving, and they engage the logical left brain more than the artistic right, the body more than the heart.
In contrast, stories depict fictional people, places, and events; they mirror the real world and are fundamentally representational. Readers and viewers act as silent witnesses, powerless over the characters and events depicted but moved by them all the same. Stories appeal more to the right brain than the left, more to the heart than the body.
Given these stark differences -- games as abstract, interactive problem-solving exercises, stories as representational, passive depictions that elicit emotion -- it's quite strange for game and story to be combined into the same experience, but Dungeons & Dragons did exactly that, and with much success.
The key was the simulation: it was both interactive, with goals, obstacles, and rewards for players, and representational, with characters, settings, and events for players to encounter. Moreover, these seemingly disparate game and story elements were often combined; a quest to save the princess from a dungeon, for instance, neatly ties a goal to a character, an obstacle to a setting.
Today the vast majority of video games, from FPSes to platformers, adventure games to open world games, are games that tell a story, and D&D blazed the trail.
Class Struggle (and Cooperation)
As influential as Dungeons & Dragons may have been in the field of story-driven single-player games, that isn't the complete picture; with its system of character classes, D&D has had an impact even on competitive and cooperative multiplayer games.
One example is fighting games, which feature a rock-paper-scissors method of balancing similar to that used in D&D. Fighting games often have quick weak characters and strong slow ones; similarly D&D has agile but frail thieves and hardy but slow fighters. Such differentiation provides balance to fighting games, where no character is vastly better than another in every situation.
Team-based FPSes like Team Fortress 2 and Call of Duty 4 also benefit from class systems. At their core, character classes and the rock-paper-scissors balancing on which they are based embody the opportunity cost of specialization -- the fact that getting really good at one thing comes at the expense of being really good at other things.
For team-based games this is ideal; a player with a soldier-type class, for instance, cannot excel at everything, so must cooperate with teammates -- medics, engineers, and so on -- if he really wants to win.
Games featuring a class system with levels have an additional benefit: they provide a sense of progression and reward as players become more powerful and unlock new abilities. Such progression acts as a carrot-on-a-stick motivation system, where the next reward -- a new perk in Call of Duty 4, for instance -- is always just out of reach.
Note that classes and levels don't even need to be explicit to have these benefits; for instance, BioShock's tonics and plasmids offer players the same opportunities for specialization and progression as other games with more rigid and defined character improvement systems.