I think S-expressions are annoying in that they tend to infect a language. If you're not careful, every function invocation becomes an s-expr, and you're left with the idea that + is no different from any other function, so we could write things like "plus(a, b, divide(c,2))" if we wanted to make a functional notation. And S-exprs don't save us from associativity, here you've made + associative, which is, fine, I guess, but what does (- a b c) mean, or (/ a b c)? This sort of asymmetry is a burden that isn't worth dealing with in my mind.

On thing that I think imperative languages bring to the table is naming intermediate values through assignment, so we can do things like

    base_index = root_index + offset
    midpoint = length / 2
    final_index = base_index + midpoint

It's rarely useful to be so verbose, but it's nice to have the ability.

(Assuming I'm understanding correctly,) I'm honestly curious why `+` _should_ be different from any other function. (Also, AFAIK the ability of naming intermediate values is totally separate from imperativeness; see Haskell's `let` form, for example.)

Still a bunch of pointless parens. Obviously

a b + c 2 /

or even

/ 2 c + b a

is the more efficient, unambiguous notation.

  a b + c 2 / +

or

  + a + b / c 2