> Certain points like the fact that Emacs isn't multithreaded are thrown around by people who don't have the intuition that multithreading isn't the right thing for a lot of applications.

GNU Emacs has a lot of applications where it is the right thing. It's no longer the simple editor. It comes with more than a million lines of Lisp code implementing all kinds of complex features: like various network client applications, IDEs, ...

> Common Lisp+SLIME shares some of this convenience, but not to the extent that Elisp truly does

SLIME is mostly implemented in Emacs Lisp. A lot of other Lisp systems can locate all source code and all documentation. It's not so much a special feature of Emacs Lisp, but its development environment. It's also not necessary that the IDE and Lisp runs in the same process / same machine to be able to look up documentation and code. It may be convenient in GNU Emacs, but any such editor can provide such features for programming language implementations.