But of course Common Lisp has similar qualities: one can still write unexported symbols, there is just the worry that the internals may change. I think the important qualities are that the configuration is typically done with global and buffer-local variables (a super-lightweight and extensible solution to the obvious problem of per-buffer state) with simple types (alists, hooks, etc).

One issue with Common Lisp style packages is that they may interfere with lazy loading. In emacs you can setq a symbol before the package loads and then the defvar will not change the value, but in CL you cannot read the symbol until the package is defined so things like eval-after-load (ie lazy load then configure) suck.