I'm intriqued that somebody went and developed a whole CAD and released it as GPL, in pure python.
The gust of why ThanCad is better, is that it has some fancy features basically all other CAD apps lack, because it was easy for the guy who implemented them because, he built it open from the ground up. It's the lesson of simplicity really. It makes sense, because for instance in autocad, when you wanted to tag on some fancy features, you mostly can't because you don't have the code, and even if you did have the code, you can't because it's big and bloated.
The architecture seems pretty much an exercise in patterns (as in gemma et. all), but I suspect it works because it's bourne of practicality (rather then academic blindness to the real world).
On the downside, the choice of toolkit and technique leads to many problems and workarounds. While the speaker enumerates the problems met with doing it, ideas pop to my head of doing that in opengl, some small gui toolkit and greenlets.
It seems a highly interesting project, I'll definitly check it out