A perfect example of what I am describing is the continual flattening of the greens at Augusta National. The old greens are unputtable at today's speeds.
I am talking about greens with significant contour. I am not speaking of slower, moderately contoured greens. I do agree that one way to make things easier for average guys and harder for experts is to slow greens down a little. Of course, slow greens mean fewer three putss but it also means fewer holed putts at distance. With SIGNIFICANT contor and greens stimping at 10-11 (very fast and with significant contour an effective stimp above 12.5) you force a player to "hit" uphill putts and be defensive on downhill putts.
That is very challenging as you encounter putts that may start off "slow" uphill and then finish "fast" by rolling away from you. Too hard and the contour makes the ball roll far away and too soft and the ball never climbs the hill. If you really want a tough combo, have the greens very firm and yet somehow not so fast
Let's face it--most goflers want soft, "receptive" greens, fast greens with small, continuous breaks. (not perfectly straight). Oh, and bunkers must be perfectly raked and "consistent" (a whole other topic)
I hate to admit it but we stimp greens every day (or almost every day). I am adamant that we do not publish "results". People in general have no idea the difference between 10 and 12. It is HUGE!
But I do think, in general, the more physical motion a stroke requires, the more challenging it is.