There is nothing to say about Fallout 1 that hasn't been said. And yet here I go saying it again, because you can never heap enough praise on a game as good as this.
Fallout 1 is the best Fallout game. Later games in the series may have had better graphics, a broader scope or a larger focus on faction mechanics, but Fallout 1 has atmosphere. It's a darker game than its sequels (not grimdark, just dark) and succeeds at creating a wasteland that feels rough and devoid of hope while still carrying a bit (but not too much) of the series' signature sense of humour and levity. In some ways it has more in common presentationally with the Metro or Stalker series than other Fallout games. It also has a clarity of vision which hasn't been seen in any Fallout game since. It's a relatively short and narratively focused experience for an open world game, but within that focused narrative we get something a lot more thematically interesting and coherent than the often disparate and self-contained side quest stories of later entries, which often felt more like a funhouses than connected, living worlds (I'm looking at you, Fallout 2 and 3).
Lastly, Fallout 1 has the best villain in the entire series. In case you're actually new to this game and not just an old head re-purchasing it on Steam, I won't spoil anything about this character. The meeting with him/her/it should be experienced blind.
What I primarely like about Fallout 1 is how much your character build matters. Many modern RPG's have forgotten the art of emergent gameplay, of having game events defer back to the player's stats and playstyle rather than just his dialogue choices. Your experience with this game will be vastly different if you have, say, a high doctor and speech skill coupled with an intelligence of 9 than if you're playing as a combat specialist with an intelligence of 2. Whole entire swaths of conversations will change, the way you approach problem solving will change, avenues that are open for one type of character might be completely closed for others or force unorthodox solutions (like using plastic explosives to blow open a door that you can't lockpick, or using the pickpocket skill to plant grenades in enemies' inventories), and so on. It lets you forge your own path through the game in a way that picking between outcome 1, 2 or 3 in, say, a Mass Effect game simply doesn't allow for. In many ways Fallout has more in common with System Shock or Deus Ex than the modern RPG genre it helped define, because it's mechanically driven just as much as narratively.
One way to illustrate how gameplay mechanics and character build can vastly change one's experience with this game is through the mechanic of crippled limbs. If you get shot in the leg, there is a chance it will simply give out, forcing you to either heal it or find a doctor who can. As long as your leg remains crippled, you can't run. At all. This sort of scenario creates its own mini-story within the game. You're now forced to potentially spend days or weeks looking for a doctor's bag, a relatively rare item, or a doctor who can heal you, a somewhat expensive ordeal, changing your entire path through the wasteland, making you postpone otherwise important, sometimes time sensitive tasks. Unless, of course, you're a skilled doctor yourself, in which case you can fix your leg and move on. In Fallout 3 you heal a crippled limb by the press of a button, so there is absolutely no weight to it. Some might prefer that, but in my opinion Fallout 1's more unforgiving game mechanics lend weight to the notion that we are in a wasteland, and not a theme park. And it gives you a wonderful sense of player agency and success when you come across a problem that your character is uniquely equipped to solve.
Fallout 1 is a very open game in the truest sense of the word. You can technically make a beeline for the final boss the moment it begins, and there is a constant sense that the game is willing to accomodate everything you (try to) do, including stupid decisions like running headfirst into a pack of deathclaws at level 2, with very little in the way of artifical barriers, handholding or forced story moments that you can't avoid. Fallout 1 doesn't make you complete a tutorial area, it doesn't tell you which places are safe or which NPC's are trustworthy and you can completely miss certain story-important people or plot developments if you're not observant. A lot of its real storytelling takes place in the background, between the lines. There is ambiguity about the different factions and their motivations. Even its inhuman villain can be understood and sympathized with, and his entire backstory has to be pieced together rather than told directly.
Presentationally and graphically, this is obviously a very old game. It's a turn based, isometric game, which for a very long time was considered outdated and passé in the industry (though thankfully no longer). Its UI is clunky and could definitely stand to be overhauled in a major way. I'm not going to say you'll have fun immediately if you aren't used to this style of game. But with persistence Fallout 1 becomes an amazing experience, one that makes you forget its age like all timeless things do.
And when it comes to certain things like voice acting it is actually much better than later entries in the series, especially New Vegas and 3 (although, to be fair, only few characters have voices). Music and sound design are on another level too. The ambient tracks that play as you explore the wasteland sound like how a wasteland should sound. There is a reason New Vegas re-used these tracks. They definitely have a ton more character than Bethesda's forgettable background pling-plong-pling compositions that seems directly lifted from Oblivion and Skyrim.
All in all, this game is the real deal. It's the one they will dig out of the burning rubble in a hundred years when nuclear war has destroyed everything. They will then conclude that human beings, while stupid, at least understood the art of designing good RPGs.