The more I learn about the SVG spec, the more I understand the rationale of some of the UI decisions inkscape made, and the more impressed I am by how they implemented advanced techniques like shape union and intersection, clipping and masking.
From the little I know about SVG, I wish there was an open source alternative to Inkscape that didn't support standard SVG but used a proprietary format instead. Almost everything you need to create vector art, SVG doesn't support. Multiple outlines in a single shape? No. Varying thickness in an outline? No. Rounded corners on arbitrary vertices? No. Non-destructive boolean operations? No. I'm not even sure SVG suppo…
Do all the others (AD, CD, AI) use some proprietary format that makes their life easier? Is there no better alternative to SVG on the open source side of things?