I've never tried the original APL, but J and q are great interactive calculators, due to their terseness and overall structure.

For example, if I want to convert a 4-item vector to fractions of the total, I can write:

  x <- c(1, 3, 2, 5); x/sum(x)
  x=np.array([1, 3, 2, 5]); x/np.sum(x)
  {x%sum x} 1 3 2 5
  (%+/) 1 3 2 5

in R, python, q, and J respectively. In q and J, I don't have to type the commas, and many of the parentheses and brackets go away.

I know it seems like a trivial thing. It certainly is for code, i.e. stuff that you're saving in a file and intending to use later. But for an interactive calculator, it's important.

Clearly, if you're not doing very mathy work, things like this don't come up. But in its domain, it's very nice.

(Edit: TCL is similar in this regard: it doesn't require as many commas and parenthesis as other languages, making it a very good, well, tool command language.)