I'm a huge fan of the parenthesized syntax.
fn(arg1, arg2)-esque syntax. Ruby also has
fn arg1, arg2 at the statement level. It would be familiar for me.
On a more actionable level, I want to start using Reason at my work, and I can't do that unless I convince everyone on the development team that it's a good idea - at the minimum, they're going to be doing my peer reviews, so they need to understand it as easily and simply as possible, and familiarity can be a big help to multi-language developers.
One recognition issue I have with the current syntax is that if I see
f g h i, if you accidentally skip the first word or are glancing across the screen and see this from right-to-left, you have to reprocess which function is the root one being applied to. This conundrum doesn't occur with
f(g, h, i) because the function callee and arguments are so syntactically separated.