Going a little further, Emacs is itself (at least a lot of it, including js-mode, for instance) written in Emacs Lisp. This makes most of it modifiable by its user. It's not impossibly hard either - I have made a minor mode for Pyccuracy tests in a couple hours. Learned some in the process.
Even the "configuration files" are, really, Lisp code, not bags of values and options.
Just to add to the language-configuration thing on the vim side...
Vim configuration is also not a bag of options and values; by default, it uses vimscript. vimscript is an interesting language (supports a form of OOP) but only really usable in the vim world, so it is up to you if you want to learn it to do things besides assign values to options.
Alternatively, vim allows you to do editor customization in perl, and while I have found very little documentation for it, it also supports mzscheme.
How does this kind of thing help anyone? Why is Notepad just not good enough? You can add entire text-manipulation features to your editor, including Intellisense to your specifications, tools for refactoring, integration with other tools outside the editor, and so forth.
Both editors have large communities and have few real competitors for fast and powerful text editing (I hear TextMate comes close).
The parts that are written in C are mainly the lisp interpreter itself, and some windowing stuff. Unless you are doing something really strange you never need to dive into the C internals.
Even the "configuration files" are, really, Lisp code, not bags of values and options.