Multiple dispatch is a big one. Also, macros, which are what (the view layer of) CLOS is built with. In most other object oriented languages, the language isn’

Multiple dispatch is a big one. Also, macros, which are what (the view layer of) CLOS is built with. In most other object oriented languages, the language isn’t powerful enough to implement its own object system. Having the full power of function calls in all cases (named or anonymous) is incredibly helpful. I know the Python party line is “just use def” but adding a line here and a line there adds up fast. A syntax for function objects is also great. I’ll also call out the ability to use dynamic or lexical scoping on a per-variable basis. That has saved me hundreds of lines of work, and made an O(1) change actually an O(1) patch.