IMO, the language is only a small part of a deeper relationship with your IDE. The IDE needs a good debugger, a good introspection system, and good hooks to mod

IMO, the language is only a small part of a deeper relationship with your IDE. The IDE needs a good debugger, a good introspection system, and good hooks to modify things (ideally anything). Emacs meets this criteria, but few others do. This is why there is a living corpus of 40+ year old elisp that can be drawn upon for functionality and inspiration.