Consistent APIs for collections
I have been using Clojure a lot for work this year and the consistent API for anything that is a seq (lists, vectors, maps, trees, etc.) is probably my favorite language feature. Scala 2.8 collections offer the same uniform API. For me Clojure and Scala, with a fairly small number of operations to remember across most collections therefore represent a new paradigm for programming compared to some older languages Like Java, Scheme, and Common Lisp that force you to remember too many different operation names. The Ruby Enumerable Module is also provides a nice consistent API over collections. Most Ruby collection classes mixin Enumerable, but the API consistency is not as good as Scala and Clojure. That said, even though Enumerable only requires a small number of methods to be implemented like each, map, find, etc., the ability to combine these methods with blocks is very flexible.