Some effects, such as the Barbarian's Overpower ability, gain benefits from striking critically a lot (in this case, the cooldown of the skill is lowered by one second per critical strike).
The answer to this question would have some important implications for certain builds, which focus on these abilities. For example, one-handed weapons would clearly be better for such builds, since critical strike frequency is higher with them.
The question is: What is "one critical strike"? Are critical strikes rolled per enemy, or per attack (meaning, if you crit, do you hit all enemies critically?)? And then, are they counted as individual critical strikes per enemy, or only once per attack?