The biggest reason it failed for me was them changing things from KSP1 that didn't need changing.
I'm big into building rockets, less big into long and intricate mission, and to me the biggest hindrance to my enjoyment of the game was all the stuff they changed in the VAB.
Fairings:
I cannot get used to the new way to build fairings. It's buggy, takes way too many clicks to use and refuses to let me close them whenever I tried to use them as interstages or the like.
Also I cannot seem to find a way to disable angle snapping on them so I cannot make smooth gradients because it always snaps to 90 degrees when I get to close.
This more than anything has been an absolute dealbreaker.
Having it open on mouse-hover also works far worse than in KSP1.
Staging:
It's also buggy, sometimes I just doesn't react to my mouse clicks or doesn't swap stages when I'm trying to move them around.
The readability is also just way, way worse. Having the actual stages float with your mouse when you move is infinitely better than the red and green dotted lines that KSP2 has.
I have no idea why they'd change this.
Parts Manager:
It has a place in the game, but NOT as the right-click menu.
When I right-click on a part, I want the options for just that part (and it's symmetries), not the entire part list. I slows down performance and massively clutters up the screen.
It's also annoying having to close it every single time manually.
Camera Controls:
I prefer the controls of KSP1, but that's just a matter of knowing them by heart after 2k hours playtime. The issue with KSP2 camera controls are just that they're so buggy, with my camera randomly re-centering on my craft whenever I don't want to.
Craft Saving:
I thought the old system was simpler and more streamlined in usage. I massively prefer individual craft files categorized by folders instead of cramming multiple crafts into a single workspace.
Part Variants:
I sorely miss part variants, especially the Bare engines because they make building good looking rockets so, so much easier and in general make rockets look better when combined with Engine Plates.
1.875m Parts:
By far the best parts to build good looking rockets with imo. The gap between 1.25m and 2.5m is just too big and these parts fit perfectly in between. I hate that they didn't add this size into the game.