The resolution of the output image in the case of PNG is something you can specify, so you can make it as large as you want.
edit: "render" -> "export"
Perhaps even better would be a set of instructions that could be piped into some other program to transform into whatever it needs.
That is to say, if you could create a simple parseable format or use an existing diagram lanuage, if one exists sufficiently compatible, then those who want to convert it into PSTricks or SVG or whatever could then base their conversion off your language. Both PNG and PDF don't seem amenable to such a transformation.
One use for this could be using your program to create a basic figure template for some math problem involving a diagram, one whose parameters might change and thus some of the textual content changes without necessarily changing the text. This could be a SVG on the web, for example.
It also would allow for a better integration with particular fonts, etc., in embedding into another document, such as a LaTex document.
Perhaps, at a minimum, since you have a scripting language you use, you could have a diagram to export it as a whole diagram as something in that language. That might be a bit dangerous since ideally your exported language format would not change much and I could see your underlying scripting language would change. Also, ideally the export language should be rather explicit to make it simpler to parse, e.g., a loop to draw concentric circles could just output the concentric circles in the simpler language while the Lua script would naturally be having the loop.
Perhaps I misunderstand your question though. Are you envisioning an export format with more semantic information about the diagram, rather than just a simple declarative description, as is present with SVG or Postscript?
I see now that it says SVG in the body text. I totally missed that the first time. The heading of "export to pdf" along with the graphic that showed PDF and PNG support in the drop down was what filtered into my head.
Now I just need to wait until it comes to the Mac. Looking forward to it.