Fantastic idea for a thread.
It would be great if we could come up with some sort of "Theory to Track Design", and this is a great starting point.
How about looking at seeing if what we consider a "good" track to race on can be quantified? Would it be possible to come up with some sort of analysis that allows us to say a certain race-track is quantitatively the best in the world. You could have certain "indicators" such as what Purist has suggested, as well as things like corners per mile, and average radius of corner (or something similar).
Corners per track could be a more easy one to start off with.
There are quite a few obvious obstacles in the way of course, but it might be nice to see what people can come up with.
I look forward to seeing what your other topics are Purist!