Hillshading less visible on higher zoom levels
When zooming in with hillshading=on, the contrast of hillshading is less than on lower zoom levels (decreasing with zzoming). Because of this, sometimes hard to find exactly the bottom of a valley, or the ridge/summit of a hill.
On zoom level 17 hillshading totally disappears.
In fact, I think it would be better to see a higher contrast in higher zoom levels, because it is more important to see the smaller differences in the altitudes of the terrain.