Actually I *did* start using Emacs because of the runtime configurability and extensibility (and learnt Lisp as a result and fell in love with it), but I know I'm weird.
Its implementation language, of course, is partly ugly old Lisp and partly really rather nasty C (GCPRO, anyone?). The less that's in C and the more that's in Lisp, the better, as far as I'm concerned, but not because of the language -- more because, if it's in C, I can hack at it interactively, whenever I need to. :)