This is simply nonsensical. Even the applications you listed don't just "do one thing". Vim and emacs have a trillion features each. Git has way more features than some other versioning systems. When does "one thing" turn into multiple things? Within the context of the thread - when did gmail stop doing one thing? When they added filters? When they added all of the labs features? With this current announcement? Do one thing and do it well is great but defining the scope of the one thing is not something to be taken trivially.

What is the one thing Emacs does well?

Emacs is the polar opposite of the Unix philosophy; it's more in line with the Lisp Machine tradition. Programs like sort, uniq, and grep don't embed interpreters for dynamic languages and provide thousands of different extensions for anything from reading mail to connecting to IRC.