You get access to things like org mode and magit and deep REPL integration and an integrated package manager. You get the ability to configure absolutely everything in elisp. Many experienced Emacsers routinely write small snippets of elisp to automate some task. This goes well beyond adding a key bindings here and there, or the odd macro, and is really amazingly powerful.
No. This has been discussed ad nauseam already. Vimscript offers a limited API to some functionality of Vim. Emacs is written in Elisp, and thus, everything is accessible and changeable in Elisp. What is more, Emacs and Elisp were designed from the ground up with malleability in mind.
