Vim needs to be compiled with Ruby support for this to work. If you're on a Mac, the default installation of vim is -ruby, but it's possible to run MacVim (which does have Ruby support) in terminal if you want to.
Is it really that difficult working with vim's own scripting language? Or does it limit you in some way?
Requiring plugin users to have a ruby/python/perl enabled vim automatically reduces the potential userbase. And one of the benefits of vim is its widespread availability.
It probably isn't that difficult but why learn yet another language (vimscript) when you already know Ruby,Python,Perl,Tcl and Scheme? All of which can be used.
IMHO the use of an embedded interpreter really only pays off for plugins that perform a more or less complex task like parsing or something that requires functionality that VimL doesn't provide like internet access or interprocess communication etc. For simpler tasks, I'd suggest to stick with VimL.
Great. I've always wanted to write plugins. Would really appreciate if this could be followed up with a few more samples with some more functionality. Such as opening a small input window at the bottom for input and one for selection (as say in Command-T or fuzzy file finder).
Nice writeup though.