I'm not a Lisp connoisseur, but the parts of GNU Emacs that bothers me are more all a) the C-based glue... single-threaded single process monolith b) giant ball of mutable state that seems to let anything step on anything else and break things in confusing ways.

IMHO there should ideally be a minimal shared transactional blackboard space that processes can coordinate in and then each buffer should be its own isolated process/thread -- like a chromium tab.