(Posted without reading everyone else's input to prevent them from influencing me, I will read through them all when I have provided me input.)
From my perspective Fallout is a very special game to me.
Fallout 1 & 2 provided me with the opportunity to explore a diverse environment, engage with unique characters, and even engage in a little archaeology Indian Jones style to shine light on what little remained of the old world prior to the great calamity known as the Great War.
For me the key attribute to the games is there capacity to allow players to engage with the game with very uniquely flawed characters that in their own way influenced the game to play in a completely different way.
Needless to say the amount of player agency afforded to the player was unheard of in video games outside of the typical D&D realm of titles.
Another details that truly interested me was the role playing aesthetic without the over the top "magic" or fantasy environment, instead I got to enjoy a dark diesel punk game layered in pop culture and dark humor. I have to give credit where it is due in this case as the developers from my opinion managed to cultivate player experiences in both a normal and bizarre way without the transition from the two becoming too jarring. The result from what I experienced was not immersion breaking or disconnecting in any way, for me it actually reinforced the overall game and my immersion.
I rarely encounter games crafted with such deft thought, particularly in today's market. Fallout 1 & 2 for me are like these bizarre intellectual games not intended for the average consumer, but for those that wanted to weigh options, plan, and even find creative solutions to normally insurmountable obstacles. By allowing me to exercise my brain Fallout managed to keep me enthralled throughout the entire experience.
It's also very rare to encounter games with such interesting world building as well. Ever notice the games with the best world building tend to have the most interested fans? The universe in which Fallout exists is this enigmatic ever ongoing investigation mired in sin and tainted by the occasional freak technology. Allowing players to explore the rich past, while explore the "status quo" that has come to be presents players with an interesting duality.
In this case the player is perfectly aware of what was, what is not, and what may be. Provided they can actually succeed in their task. Plus from intro to end the player is given a clear overriding goal that dictates the flow of the game.
The themes used within Fallout presented players with a very unique opportunity for exploration. When games are designed the ideal level of interactivity is to the maximum. In Fallout 1 & 2 players got to explore the ethics of existing in a post apocalyptic wasteland. The player could not only be the paragon of light, or the reaper of souls, but could find themselves in unique dynamics that would influence the game.
A great example of this is groups and sub groups that could be encountered in the game. Such as the guild of thieves, the desert rangers, slavers, etc...
Players are introduced to the light and dark on every level, from traumatized children fleeing from a power armed wearing individual, to helping some stranger pay rent. The spectrum of morality is grand, and through exploring that players or their characters can grow.
So in conclusion, why is Fallout special to me? Because it's a game made for adults, to be played like an adult, and requires those that play it to consider their choices rather than blinding barreling into a shooting gallery hoping for the best. It's a thinking game, a mature game, and surreal with it's introspection.
