Pat: A few years ago I would have agreed with your initial query but now I don't. Instead, I believe it is the more subtle little breaks in a green which have disappeared due to USGA construction.
Building big contours within the green is possible to do on USGA greens if you are conscientious in following the subgrade, because it's easy to tell where the big contours are by probing through the greens mix as you build. Most of my best severe greens are on non-USGA construction, but Stonewall's new course and Sebonack have some pretty severe greens and they are USGA spec.
However, what you miss in the USGA greens is the little bumpy stuff you find on many old greens. It's hard to build those at subgrade level and even if you do, that inch or two may be lost when you're putting the gravel and sand layers on. More importantly, to build a USGA green properly you should not add a little bump here or there at the end of the process, but on a non-USGA green there's no problem doing that, and that is how many of those contours have been made, whether it was by C.B. Macdonald or Bill Coore. We've got them at Pacific Dunes and Barnbougle and Lost Dunes, but not at Sebonack.