A combination of everything mate, i was never a fan of road cars either. Always considered regular AC a bit lite weight, the world scale, weight of the cars, a bit toyish. ACC came along with a more weighted grander scale, i was absolutely and have been for the last 4 years all over that. The only reason i popped back in to ac was to check it out on the pimax, my opinion on it has totally changed. Now i feel the world scale, it's a night and day different experience. I think that's a combination of looking through the bigger front screens of road cars, the horizontal fov that goes all the way up which 'really' opens up the elevation and gives you a real sense of depth and scale. There are some parts of the Nordschleife where you drop from a corner and see the upward elevation that just look incredibly real. Another thing i think is the CSP and the work in that that's just made the world so much more believable, the shadows, the sky box, trackside foliage, when coupled with the pimax and it's clear monitor like display holding frame rate gives total persistence.
Honestly, if you know anybody local with a pimax with AC setup in a rig, get round and prepare for VR racing 2.0, gone is the helmet cam.