Along with Quartz Composer, a number of other node-based graph editing environments have been very inspirational. MaxMSP, vvvv, LabView, and Grasshopper have all provided for great metaphors to learn from and extend.
We decided to launch with just 6 nodes and just a single runtime, from a working total of about 50 nodes spread across 4 different runtimes, including an audio engine built using the Web Audio API, which, funnily enough, was the first runtime.
The theory was that by removing features for the initial launch we could try and polish up a much smaller set of features.
BTW, as a sort of easter egg, all of those additional nodes and runtimes are there... I'll leave that as an exercise for the hackers out there to discover them! We plan on formally exposing them in the coming weeks.