The thing that pulls it all together, I think, is how well suited his solutions to diverse problems are. There aren’t a bunch of moving parts. Prototype was very much a library, and not a framework. His code is very much of the tool, playing well in the environment, be it Ruby, CoffeeScript, or even shell scripts. rbenv, ruby-build, and pow all play to the strength of bash and Node rather than trying to extend them to become something they’re not.
I was tempted to say his work is minimalistic. On second thought, I think it’s understated. Look at his website or photostream. The quality of just enough, but not too much, isn’t luck. It’s Sam Stephenson’s calling card. I love it.