Erlang. Even though some IDE's are starting to support it now. Erlang is the original reason I got into using Emacs actually.

Not really a programming mode.

But OrgMode is probably one of the most powerful and unique emacs modes. There is nothing like it anywhere else.

Once you go OrgMode, there is no going back.

I was mostly thinking of Agda[1] and, to a lesser extent, similar languages like Coq[2] with Proof General[3].

You could write Common Lisp in Notepad if you really wanted to. I do not think using Agda would be at all practical without Emacs.

[1]: http://wiki.portal.chalmers.se/agda/pmwiki.php

[2]: http://coq.inria.fr/

[3]: http://proofgeneral.inf.ed.ac.uk/

There's decent Common Lisp support in other editors. Lots of people use Vim with Slimv. Clojure is still supported better (especially now that we have tpope's fireplace.vim), but CL is decent to edit.

I was thinking maybe gp was referring to (at least) elisp.

Emacs Lisp