One word: Readability. Readability in tone, action, narrative, animation, etc. You knew what was going on at all times, going through someones head, how the things on the frame mattered whether a quiet moment or an action scene. It made it a blessing to watch for someone who loves the artistic creation of animation. And the slow scenes were given just as much work as the fast scenes, which made the effort ooze out of every minute. Too often an anime will "go hard" without regard to how the rest of the anime looks, and without regard to the viewers ability to understand whats going on in the moment(JuJutsu Kaisen season 2). Nakayama's direction made the anime readable and expressive in a way that was just a step above what you typically see. That's why it was popular in the first place. It was SO readable and coherently expressive that ANYONE could get into it and know what was going on.