I have found s-expressions easier to edit with a smart editor than other syntaxes. Having to think again about the layout of syntax doesn't sound very appealing, but I would probably give it a try if someone implemented it.
Though, normally, you might write something akin to:
(map (fn (nums) (+ 5 (apply + nums)))
It's a lot like Zak's suggestion, just noting that sometimes n is infinite. Full partial application is really powerful -- especially with the general trend in Arc to have the final argument be the most dynamic one -- and I regret that I don't really have time at the moment to make an interesting example.