Author of DocPad here. For a bit of history, DocPad was created 2 years ago now out of my utter frustration with all the other static site generators - frustration from not being able to use it for anything besides trivial use cases (Jekyll is the worst offender here). Building DocPad, it's goal has always been to set you free, ensuring you never hit another brick wall or constrain yourself. That's why we're seeing it used for such an awesome wide range of use cases easily and freely - personal sites, company sites, huge corporate sites, web applications, chat servers, whatever. Because of its plugin system and that it is built with express.js, you can extend it easily as well as add dynamic (re-render on each request) abilities when needed. Well worth checking it out if you got a while, here is a great intro video to it: https://vimeo.com/53755097
My advice to other people considering making a static site generator - please don't make another compiler or caching layer just like the rest, anyone can do that. But if you want to make a large difference, one that you can't do alone, then see if you can team up with an existing project, they'd certainly welcome your insights and probably have a ton of ideas already - the conversations will always be valuable for all parties involved, and the end result for the world will be much better quality solutions rather than just merely more solutions.