In my mind, D3 is a DSL for ingesting structured data and outgesting DOM. While it has certainly been a few years since I touched it last, what I remember was that it basically didn't touch anything in the data or DOM, it just made the piping between the two super easy. So I think it was that approach: not introducing new idioms for existing things, but instead introducing the pipeline approach, with a very simple syntax, all made it very amenable to learning very quickly and rapidly iterating an idea, tweaking it until you get just the thing you want.