The Universal Principles of Design (among other places) talks about the 80/20 rule — but Emacs is so powerful and customizable that no two people ever agree on which 20% is the most important!
Well, Emacs is very much "a day to learn, a lifetime to master". I've been using it for years, have led sessions teaching other developers, and just yesterday I discovered rainbow-mode (http://julien.danjou.info/rainbow-mode.html). Before that, it was magit and halp.
The Universal Principles of Design (among other places) talks about the 80/20 rule — but Emacs is so powerful and customizable that no two people ever agree on which 20% is the most important!