As far as I know, there are no explicit diminishing return (DR) curves built into the game’s formulas – mathematical formulas for damage, resistances, healing etc. seem to be largely simple and linear (y = a * x + b) or inverse linar (y = a/x + b) (a and b being constants, x being the stat, e.g. power, and y being the desired result, e.g. damage)
That said, there are “natural” diminishing returns:
Once you’ve reached 100% crit chance (or 80% when you’re running with high Fury uptime), stacking more Precision doesn’t make any sense.
Stacking Toughness will make less and less difference the higher you get: Adding 100 Toughness when you only have 100 Toughness to begin with will make far more of a difference than when you already have 2500 Toughness.
Given a high crit chance, it may be better in some circumstances to stack Ferocity than Power in order to increase your average damage output.
