Wow, that's awesome to hear about the polish. We move pretty fast, and sometimes I worry about skipping over important details.
To get what polish we do have took a few things:
1) My cofounder and I have been working together for a while and our workflows compliment each other. The time we save with development usually gets spent on polish after shipping.
2) This started as a side project for us, so we made a strong effort to find a concept that was inherently simple. For example, the simpler we can make the UI, the more users can concentrate on your ideas. So our motivation towards the UI isn't to add more, it's to refine down when possible.
3) I use Precursor everyday now, so it's important for me make sure things behave well :)
We're not heavily-dependent on svg, it would be possible to render to canvas or just to straight html. But svg has some niceties, like transforms on groups and easy export to images. It definitely has its challenges (e.g., text formatting, Safari bugs, etc.) but we get a lot of things for free, and it helps keep sharing options pretty simple. Luckily our visual aesthetic is pretty minimal, so we don't have to push things very hard on it.