I read the J Incunabulum before encountering this, and the point that stands out is that you don't start by jumping into the middle of it like many programmers who are familiar with C will do; the macros defined at the beginning will confuse you otherwise. They also build upon previous ones, so the code ends up climbing the "abstraction ladder" very quickly. I personally like the Iterate macro (i), for how it compresses a relatively verbose loop into a single character; and of course in an array language, the iteration is entirely implicit.

In other words, I believe the reason this code is hard to read for many who are used to more "normal" C styles is because of its density; in just a few dozen lines, it creates many abstractions and uses them immediately, something which would otherwise be many many pages long in a more normal style. Thus if you try to "skim read" it, you are taking in a significantly higher amount of complexity than usual. It needs to be read one character at a time.

As someone who has spent considerable time working with huge codebases composed of hundreds of tiny files that have barely any substance to them, and trying to find where things happen becomes an exercise in search, this extreme compactness feels very refreshing.