One interesting design choice in Emacs that I haven't seen in other editors: reserved shortcuts.
An Emacs extension shouldn't use F5 through F12, or Ctrl-c LETTER. This lets users configure their own shortcuts. https://www.gnu.org/software/emacs/manual/html_node/emacs/Key-Bindings.html
Does it exist elsewhere? I miss it in VS Code, where e.g. all the Fn keys are already assigned.