Here's their FAQ answer for that: https://just.systems/man/en/what-are-the-idiosyncrasies-of-make-that-just-avoids.html

I imagine, there's various smaller differences in how the recipes are defined, but yeah, this is the big one.

I find that Just feels a lot like just a collection of script files, with built-in error handling for argument parsing and during execution. And it also offers easier discovery of available tasks and how to use them.