Stamina

Related:
* Fatigue

Contrast:
* Autorun

Possible uses:
* Prevent sprinting for unnaturally long periods (usually characters run or jog by default)
* Make carrying insanely heavy loads of items impossible or at least slow and aggravating
* Make melee combat more realistic by forcing the player to go on defensive (not having enough strength left to swing their weapon with enough force)
* Preventing player from attempting to avoid being harmed by continuously hopping
* ... and generally improving realism

Players often find it unrealistic that the character can "run" only for so short periods of time, but they fail to realize the character by default is running or jogging and the thing they refer to as "run" is more like sprinting. Otherwise negative comments tend to come from those who wish for more fast-paced action.