It's the syntax. The elementary nature of s-expressions makes it fairly trivial to mangle them freely, and define all kinds of clever tree structures and so on, all in an easy and consistent way. It also means you never have to remember much symbolic weirdness, you just have to remember (operator atom atoms ...), and that's it.
I think only the functional languages like Haskell and F# come close to the same expressiveness, just from very different angles; auto-currying and lazy evaluation save a lot of boilerplate, and the former is tricky to do in Lisp for precisely one of the reasons it's so flexible (easy variable arguments).
It's a little more than syntax to me, even though it's part of it. Maybe a ~rapprochement between syntax semantics evaluation of recursive domains/categories. The fact that the construction of data through sexps belongs to that same philosophy as valuation between any domains. That the same recursive idea permeates through the whole system.
I think only the functional languages like Haskell and F# come close to the same expressiveness, just from very different angles; auto-currying and lazy evaluation save a lot of boilerplate, and the former is tricky to do in Lisp for precisely one of the reasons it's so flexible (easy variable arguments).