At the time my own answers in C would take dozens or even hundred of lines. I didn't know J existed and thought people posting those absurdly small solutions were just trolling or running a scam.
Man was I left dumbfounded when I learned this thing actually runs and produces the correct answers.
For the lesser-gifted like myself, I wonder if one couldn't attain the power of J without the cryptic syntax. Haskell, for example, has a general very easily understood syntax for function applications: if "add a b" invokes a binary add function than "add a" is a unary function that adds a and the syntax allows for infix expressions, like "a `add` b", "(a `add`)", and (`add` b). (The latter more useful for non-alphanumerically named functions, like (++ [42]). I wonder if J/Apl couldn't use a similar strategy to trade a bit of terseness for something much more readable.
It probably won't teach effectively more complex features, you might need a bigger project for that. But it will add to familiarity. Where other languages have lots of libraries, J has language where many things are just so easy to assemble from few language primitives.