The lack of parentheses around function calls makes it hard for people not familiar with ML-style languages to tell where the function calls are and what the arguments are to each function call.
There is a wide variety of syntax among mainstream languages, but basic function calls are pretty much all the same.
Take a look at Reason [1] for an example of how a functional language (OCaml in this case) can be made more familiar.
Ruby has optional parentheses around function parameters (usually it's recommended to avoid parentheses if possible). Shells have no parentheses around command arguments. These languages are mainstream and no one complains about this.
Of course, if we adjust "mainstream" so that it refers to that set of languages which have pass-by-value, strictly evaluated arguments, with no implicit currying, then your point is nearly self-evident. :)
