I talked to someone about the extensibility of emacs, but the person I was speaking to assumed that any IDE is just as extensible by using Plug Ins.
Without turning the conversation into a university style lecture, what is one or two simple actions I can do in emacs to show someone what separates it from other IDES.
It is not so much about other tools or languages not being extensible; most quality ones are extensible. It is about how easy is to extend it since the text editor, debugging tools, and some other useful stuff like shell integration (process control, I/O), networking for example, are built into the extension language.
It is very easy to write a small tool that automates something, like a say project creation for Emacs compared to for example Eclipse, Netbeans, or VisualStudio. Since Emacs GUI is in essence a glorified console, it is relatively easy to integrate CLI, text-based tools with Emacs.