The ability to freely mix programming paradigms and to bend the language towards the problem instead of the other way around. It's the thing that Common Lisp is kind of famous for.
Depending on the programming sub-sub-task that I'm working on right now, I can have simple state machines written with explicit GOTOs, algorithms working on immutable state and written in functional style, minor operations in these algorithms implemented with a functional shell but imperative core, the ability to do declarative programming and define my own syntax for the declarations while I'm at it, and the ability to always branch off to a task-specialized DSL whenever it brings direct benefits to the table.
Depending on the programming sub-sub-task that I'm working on right now, I can have simple state machines written with explicit GOTOs, algorithms working on immutable state and written in functional style, minor operations in these algorithms implemented with a functional shell but imperative core, the ability to do declarative programming and define my own syntax for the declarations while I'm at it, and the ability to always branch off to a task-specialized DSL whenever it brings direct benefits to the table.