Elisp in replacement string
Something I've never really paid attention to, and it's actually quite a powerful feature: when doing search/repalce in Emacs you can replace with Elisp code (as in, Elisp code can provide the replacement).
What Emacs got right, or how I wish modern apps were more like a 50 year old text editor
Someone's love letter to how everything is a command in Emacs. I tried to sell this approach for Textual, especially when the idea of a command palette was first raised. Sadly it never came to pass.
I think it would still be an excellent idea.