The biggest thing, to me, is Babel: it's a system that lets you combine documentation and code into a single unit, and lets you combine code from many different languages into a single tool for processing. Now, instead of an unintelligible readme with an undocumented handful of scripts, I've actually got real documentation and an entire project basically in a single file.

http://www.jstatsoft.org/v46/i03

I've been thinking of starting a project specifically in org-mode for quite a while.