Definitely first-order functions made LISP powerful compared to its contemporaries-- but also you can't mention abstraction without mentioning macros. Macros ar

Definitely first-order functions made LISP powerful compared to its contemporaries-- but also you can't mention abstraction without mentioning macros. Macros are first-class metaprogramming, and very much enabled by the uniformity of s-expressions. That said, people can do awesome stuff with any language, so all this is very subjective IMO.