Ran suggested a few days ago that the greatest difference in the Doak scale in between 9 and 10.
Allow me to suggest that the most profound difference is between 8 and 9.
Reasonable minds can always disagree as to whether Pebble, Sand Hills, Merion, Pinehurst, Newcastle, Dornoch, Muirfield, NGLA, Shinne, and a few others that slip my mind, are either 9s or 10s.
There are far fewer examples of 8s that could be considered 9s. We would most all agree that Portmarnock, Lahinch (especially since it has been changed for the worse), Five Farms, Yale, LACC (where I play), Baltusrol and a few others, are 8s, but few would elevate them to the above class.
Perhaps, one could make a credible argument that there are courses that fall somewhere between the 8 and 9 (but there are far fewer examples) -- perhaps, Riviera, SFGC, Somerset Hills, Turnberry, Carnoustie.
Nevertheless, the material dividing point seems to be between 8 and 9. Others thoughts?