CLOS looks really interesting. So many good ideas. A lot that I have been thinking about for a while.

https://lispcookbook.github.io/cl-cookbook/clos.html:

arguably one of the most powerful object systems available in any language.

https://en.wikipedia.org/wiki/Common_Lisp_Object_System:

Another unusual feature is that methods do not "belong" to classes; classes do not provide a namespace for generic functions or methods. Methods are defined separately from classes, and they have no special access (e.g. "this", "self", or "protected") to class slots.

Love this.

And the multiple dispatch.

:before, :after, and :around

Guess these are similar to aspect-oriented programming. Maybe useful for plugin systems.

change-class

This looks pretty cool for GUI apps. So many times you want to change a component to something else.