Over at the Emacs reddit, Gink0 asks What dev feature is available in emacs but not in the current mainstream IDEs? It’s sort of like the usual question of “What makes Emacs so great?” or “Why do you recommend Emacs?” but with the added constraint that it has to be things that other editors don’t have.
For me, the salient feature is that Emacs provides the closest thing we have to a modern light-weight Lisp Machine. It’s not the “Lisp Machine” part that matters, of course, it’s what “Lisp Machine” is shorthand for: a complete, totally, configurable, and even reprogrammable operating environment. That means that can do just about anything you do on a computer in Emacs and do it with a unified interface.
Many of the responses to Gink0 hinted at this but emphasized the configurability that Emacs has far in excess of other editors. Other responses mentioned Org mode and its enabling of literate programming. Other editors may support literate programming to varying degrees but Org mode’s code blocks are special. Not only can you have traditional literate programming in the Knuth sense but you can also embed executable code in an Org file and have the results of running that code appear in the file. It’s tremendously powerful regardless of whether or not you like Knuth’s version of Literate Programming.
Those are the big things. Some of the commenters mentioned smaller things such as narrowing, rectangular editing (although lots of editors have that), the best Vim emulation, undo tree, Eshell, and the notion of modes.
What’s on your list?