LuaLaTeX as a nervous system
Somewhere between the macro expansion and the rendered glyph, a document stops being yours. LuaLaTeX just makes the nerve endings visible — every \def a synapse firing in the dark, long before anything reaches the page.
What I keep noticing is the lag between intention and appearance — you write a command, and it travels through three or four layers of expansion before it ever touches the page. Most of the time you don't see any of it. You only notice the system exists when something misfires: a spacing rule collides with a font feature, and suddenly you're reading a stack trace instead of a paragraph. That's the moment it stops looking like typesetting and starts looking like a body.