Without a lot of control aside from the "Add Test Entity" call and editing the direct json text, it's a bit daunting to use in its current state. However, I can see the value in making something work right from the web browser and the proof of concept here is strong.

I think with a lot of effort on the 'user-friendly editor' side of things, there's a strong potential as a DIY online game maker here. I'd recommend looking at games like The Incredible Machine or Contraption Maker as inspiration for how to do a world editor.

A great idea and technical proof of concept, though!