I think the instruction set, or the minimum number of primitives you need to implement in the "host" not the language itself is fascinating.

I've looked at a lot of implementations that have a DROP definition like this, for example:

    : drop ( x y -- x ) dup - + ;

That works UNLESS your stack has only a single value on it. Things like this get simpler if you have a variable pointing to the top of the stack where you can just change that - but otherwise you end up having to implement DROP as a built-in.

You can obviously implement "<" in terms of ">", and if you can multiply by -1 you can implement subtraction in terms of addition, but at the same time writing "-" is no harder than writing "+", if you're not targeting a small CPU like a Z80.

I love to compare implementations, to see which core primitives they've chosen, and explore the consequences. These days I think my interest is more in implementing FORTH than actually using it, but I suspect I'm not alone in that regard!