The first half of Media Consumes Me's fairly elaborate history of the Fallout series is online, which takes us on a quick journey from Electronic Arts' Wasteland to Interplay's much-despised Fallout: Brotherhood of Steel.
What made Fallout unique for it's time was the ability for the player to respond to dilemmas in the game. If you did something good, like taking out a local gangster you would earn good Karma points, or if you did something bad like become a slaver you earned bad Karma points. This pretty much made the game extremely replayable as there was a variety of different outcomes and multiple endings to the game.
Your character's intelligence also decided how well you interacted with NPCs, something you wouldn't normally find in other RPGs. For instance, your dialogue would be a series of grunts and unintelligible gibberish if your intelligence was less than 4 points. NPCs would sometimes take pity on you, and others would totally ignore you, making side quests almost impossible unless you had Mentats, a drug that improved your intelligence and perception. Drugs played a large part in Fallout's universe. Instead of potions or spells like in many other RPGs of the time, Fallout incorporated drugs as a way to temporarily increase your characters main stats, but take them with caution as your character would feel side effects and could potentially become an addict.
Your map, quest log, and other options were handled through your handy Vault-Tec Pip-Boy. It made the game's interface feel much more in tune with the game world, and provided all the information you would need. Traveling in the map screen, you would stumble upon all sorts interesting encounters, like fights already occurring, caravans, dead bodies, and many of the game's in-jokes and science fiction references. It made the game world more dynamic when traveling from town to town.