I personally love the series because of,well,everything. It's just one of the finnest examples in gaming (yes,I'm including 3 and NV in here).
I love the setting(Mad Max fanboy here),the music,characters,gameplay,humor,writing.
It just blends together nicely to make a series that I think that everyone should enjoy and just the stuff you can do in the series (POS does no count) it's just mindblowing same go for the replay value.
Thank you Black Isle and may your soul rest in piece and your memory shall live on here and other projects by Obsidian and Inxfile.

For me, it's the freedom of choice. Fell in love immediately, way back when.
The ability to just say screw you and your stupid tasks and draw a gun.
And the game doesn't break, you just move on.

Almost always ended up wasting all of Morodinos, bishops and Salvatores,
well... except the few left sound asleep in their beds.

I absolutely loved it how Fallout force-wrenched my usual "isometric-rpg-nuh-uh" view into space. Having heard of it for years, and of how it's a super-cool game and all that did nothing to convince me - but the discount let my curiosity loose and now here I am, fondly recalling my first descent into the Glow.
I just couldn't stop playing until I beat it.
That's how it works with good movies and excellent books, after all, and I feel obliged to mention that storyline is what attracts me to any media.
And oh, the replayability.

Everything so balanced and polished and climatic, even if you hate RPG's as much as I thought I used to, chances are you'll come to love this one.

For me it's the freedom. The atmosphere is up there too. Granted, I haven't made much headway with the first two main games (I kinda have problems with perspectives), but everything I've played of the series I've loved. Most games don't seem to work nearly as hard to really make you feel like a part of that world. You can just go around, accepting quests, and it seems like there's always more than one angle in any given questline that makes you question just what the hell you're doing.

I remember the first time I played Fallout, back in '99. It was a revelation, it was like it was made just for me, the game that I'd been waiting for my whole life, what a CRPG should be -what the hell: what a videogame should be, and them some.

I was visiting some dude I'd met the week before (he is now one of my best friends, but we had just met back then) and at some point he went "Oh, so you're into RPGs, sci-fi and all that, and videogames too? Check this one out, I'm sure you'll like it", and he loaded one of his Fallout saves. "This game looks cool, may I give it a try?" "OK, but you'll have to start your own game, and don't mess with my saves" And then I just ignored everything around me and played on for over 12 hours (I'm not sure, might have been like 14 even). I left that house like a thief at 9 a.m. after having to force myself away from that wasteland... I had to play that game again.

And I did, and then I found Fallout 2 and it was the same only slightly better (and Fallout wouldn't run in my newer PCs, so...), and I kept reinstalling and playing Fallout 2 every year or two.

Why do I love them? Because of the freedom, because of the atmosphere, because of the story, because of the characters, because the character you create really influences the gameplay and how you interact with the world and the story, because every choice you make matters, because you can change the difficulty and surf the story and the world or be challenged by some pretty tough tactical battles at will... because everything, basically :)

I love it because the story and it let syou do just about anything you want. Sure games made now a days give you freedom but few can pull it off as good as Fallout. I even like 3 and New Vegas its different but different isn't always bad. It still gives you freedom to do things how you want.

That's right, and also there wasn't much freedom in games back then, was there? So Fallout gets to be a visionary input, and possibly the one to trigger the demand for such liberty.

For this reason I tremble in anticipation for Witcher III. ;)

3 and NV feel noisy and *crowded* to me, even though I spent hours wading through these really desolate wastelands. Yes, the crowd wasn't so big and besides it's only expected of humans to increase in numbers in the world, I know. It just made a serious difference. But I liked those games too. Nothing like the first Fallouts, though.