My 2¢: What I demand in an editor, in rough order of necessity, is:

1. Syntax highlighting for a wide variety of programming languages and file formats.
2. Tight integration with source code management, preferably supporting multiple systems.
3. Transparent access to local and remote files.
4. An uncluttered user interface. The exact opposite of this.

I have found no other editor which can compete with Emacs for those features. There are a couple that come close. TextMate lacks remote file access; BBEdit is also close, but the cost is off-putting.

Emacs excels at all of these, and much more. But those are the core features I can’t live without. In particular, Emacs’ SCM integration is unmatched. It supports most anything you’d want to do (view revisionlog, annotate, add, commit, diff vs. BASE, resolve conflicts, edit log message), and these are all tied together. If you’re looking at the revision history, you’re one keystroke away from opening that revision of that file, or a diff of the changes in that revision. If you annotate a file, you’re one keystroke away from viewing that revision’s diff, log message, or looking back another version. If you get a diff of your changes, you can easily revert some changes while keeping others.

It’s extremely comprehensive, and it makes working with SCM ridiculously easy. When you want to commit changes to a file, you hit C-x v v, edit your log message right there, and finish the commit with C-c C-c. It’s about as unobtrusive as you can get; there’s no need to context switch into some other app. And it works the same with any SCM: CVS, SVN, Git, Arch, BZR. Again, no context switch, like with some editors that work differently depending on the SCM.

So. There are lots of other things I like about Emacs, but those are the core things I need to be productive. And Emacs just does them better than any other editor. If you have suggestions of other editors that might, I’d love to hear them.