There are problems with Emacs. First, Elisp is kind of an outdated and dirty Lisp dialect. It's kind of neat for extending Emacs, but it's also a poor Lisp dialect by modern standards. I would love an alternative to Emacs built on top of Scheme, with its awesome continuations support, or on top of Clojure.
The other problem I'm seeing is that Emacs, as awesome as it may be, is also outdated in what it can do. Smalltalk set a high bar long ago in what IDEs can do and Smalltalk IDEs still haven't been surpassed in terms of experimental development that people tend to do in REPLs nowadays. Having something like a Smalltalk IDE, that uses realtime feedback and visuals to aid in learning, discovery or debugging, with the same philosophy of Emacs (e.g. open-source, totally customizable, turtles all the way down, transcending languages or operating-systems, etc...) would be priceless.
This is why I have high hopes for Light Table [1], hopefully it will bear fruits as planned by its author.
This is exactly why @kiwanami developed EPC (a general RPC protocol/libraries for Emacs) and why I wrote Python binding: https://github.com/kiwanami/emacs-epc
He was also inspired by the article.
[1]: https://github.com/Golevka/emacs-clang-complete-async/blob/m...
[1]: http://lists.gnu.org/archive/html/emacs-devel/2012-06/msg005...
[3]: http://vimdoc.sourceforge.net/htmldoc/eval.html#libcall%28%2...
That said, Emacs has a very rich Elisp ecosystem that's not going away anytime soon. By focusing on extensibility from the very beginning, Emacs has done many things right (it's just an Elisp interpreter with some editor functions bundled in); in comparison, Vim is a dumb-text editor focusing on keybindings, with extensibility as an afterthought. I don't think it is possible to replace Emacs today, primarily because writing a truly extensible text editor is a huge project with many boring parts, and there is no money to be made as a result. Elisp does have its drawbacks, and the Emacs community is suffering because of "philosophical" decisions like bzr (a dead project), instead of focusing on technical merit. So, the project will die a slow painful death, but I don't think that's happening anytime soon.
The takeaway is that Elisp needs to learn to interface with other languages better. There is a dead Elisp <-> Ruby bridge [5], but clearly not enough has been done about it.
[1]: https://github.com/auto-complete/auto-complete
[2]: https://github.com/Valloric/YouCompleteMe/blob/master/autolo...
[3]: https://github.com/Valloric/YouCompleteMe/blob/master/cpp/yc...
[4]: https://github.com/brianjcj/auto-complete-clang/blob/master/...
[5]: https://github.com/trogdoro/trogdoro-el4r