So, I would say the idea of dynamic redefinition is probably the big thing: In cl I can redefine a class and have all instances updated while the program is run

So, I would say the idea of dynamic redefinition is probably the big thing: In cl I can redefine a class and have all instances updated while the program is running. I would also say that the error management is also beyond fantastic. Say you run a function, and deep inside the call stack there is an error. In CL you can actually update the faulty function and just continue executing at the point of the error. For someone like me without any intellectual rigour at all, this lends itself to a kind explorative programming I really like.