The lack of tech points in every game post-1 hurts my soul.
Also, the environments feeling like their actual movies instead of rooms with skin packs.
The camera being more zoomed in on Sora in battles, and likewise him feeling small and the world feeling big.
Sora not being an over-powered demi god who can cross Traverse Town's second district in like two seconds flat, and was actually able to stop and smell the roses in 1.
Interacting with the environment, such as being able to climb vines and swim.
Animations for opening doors.
HUB world.
Sora's characterization, with him actually having real emotions and an actual arc.
SoKai.
Kairi (and the Princesses of Heart) being important in general. And how she was treated in this game is far superior to how she has been in any game since.
Sora and Riku fighting about stuff. I feel like they're almost too perfect with each other now.
The simple and clean plot. And what I mean by that is this was before Nomura tried to always have a plot twist that surprises even himself, and so he let the story of 1 come naturally (and it was better that way). Every game since then seems to have sacrificed character and good writing devices for the sake of being "shocking".
And hitting something at the same time as it hits you accomplishing the same thing as guarding, because I hate guarding and would do this instead of that in 1.
A lot of the elements they introduced in KH1, never to be brought up again: like summon gems. I feel like having those who were summon gems interact with the Nobodies could've been interesting.
King Triton's knowing about Keyblades, because that goes nowhere post-1 and I wish it had. But it's always nice seeing that scene in the original game.
SDG's growing bond.
The writers actually treating Sora with respect, and seeing him more than just an irredeemable idiot (who just yells out heart, light, dark, heart, bark) who needs to be saved by Riku all the time, as they do now. As someone on Blackdrazon's KH2 retrospective said, I feel like the writers forgot that SORA was the smart one in KH1 and made the right decisions consistently (despite how hard it was), whereas RIKU made stupid mistake after stupid mistake and needed to be rescued for it.
Keyblade's chosen's actually feeling special.
The game actually feeling really special and unique (and having that Disney charm) as opposed to the shounen anime clichés that it is now.
The Final Fantasy characters (Leon, Yuffie, Aerith, and Cid) actually being pretty central to the plot.
The game actually encouraging you to use different things in your arsenal, like using gravity to get chests.
Speaking of which: the version of aero from KH1, and gravity.
Villains like Ansem. They did just enough with him to make him really threatening and engaging (like, millions were dead because of him making the worlds fall, and I don't think any game since then has matched that threat), but they also didn't overdo it with him, and that actually added to his character.
Being able to play on Destiny Islands and Traverse Town.
The list goes on and on.
Also, I just realized that I pretty much stole everything that Frustrated Jacob said in his one video and put it here. Oops.
Edit: Riku and Kairi actually having a seen friendship with each other.