Interesting topic. I might agree that all great courses have "interesting" greens, but not necessarily "difficult." I think great greens are varied, but basically flat with gentle longer slopes. I do not care for greens with "moguls" buried in the middle of the greens, slopes that repel will-hit puts, or similar things. I draw a distinction between the slopes of MacKenzie and those of some of the modern designers, where the slopes are too pronounced, too obvious, too severe in too short an area.