I love Character Creation in Classic Fallout and New Vegas.
In terms of gameplay, it gives you so much variety that you're encouraged to try different builds. I think there are so many archetypes (i don't know what word to use) you can try in Fallout (really, not just the ones I mentioned), It encourages you to play a new save and see what's new, how the game changes depending on the character you've created. The world remains the same but reacts to you in different ways... as if you are a different person, hey! (and also, the world CHANGES because of you. man do i love reactivity i'm such a sucker for reactivity)
What I particularly love about Classic Fallout and New Vegas is not only their existence of S.P.E.C.I.A.L., Skills and Perks, but also Traits. Each of the mentioned features can change the gameplay in distinct ways, and I especially like that New Vegas considers them (such as Science checks, Perception checks... and so on), taking into account the character you have chosen. Or if you want to make it fancy, the game aknowledges who you are.
But Traits, man do I love it when games give me a feature with their advantages and disadvantages. It's such a good one especially for a RPG. Personally, I like to take my time creating my character; I have to think a lot about whether it's really who I want to play as, and Traits makes me take a little longer when making a character (It might sound weird, but I like that). I might love the advantages but dislike the disadvantages, and I have to calculate if it's even worth it.
Aside from gameplay aspects, I also love the sheer variety it offers in terms of writing. I've already discussed this in a previous post, but, I love how much diversity there is in each character built. I absolutely love thinking first, why does the character I'm creating has TAG skills of energy weapons, science, and repair? Did they have a pre-existing fascination with technology? Did they learn it from the limited education available in the Fallout universe, or did someone personally teach them?
I think what I mean is, I love writing characters and seeing, based on what I've written, what makes them S.P.E.C.I.A.L., what Skills, Perks, Traits do they have. And honestly, viceversa! One of my Couriers has to do with just... the first build I ever made, and then i just wrote him based on that. And i think i mentioned it, but i love how each built can be different from the last one you made, it's, again, a motivation for you to even try the games for a second time.
Extra reason why i like Character Creation: It's so silly to like, take a character from an absolutely different game and just see how they translate to Fallout.
Oh and it will be forever be my favorite thing dialogue checks where if you don't have the requirement, it says something so different and nonsensical