You know, like when someone fails a save against a fear spell and is forced to run away or when they fail against dragon fear and are forced to run away (or at least get so scared that they get minuses to their attack rolls, damage, and saves). Or when they fail against a charm person and have to treat the enemy like a friend or when they fail against a hold person and are forced to stand still.
Or when the PCs intimidate an NPC and they have to act friendly to the PCs for as long as they are present. Or when the PCs make a high enough diplomacy check and make the NPCs friendly to them.
I personally much prefer SOME NPC reactions to be governed by in game abilities rather than entirely up to me as a DM. I have enough to worry about when running a game than having to make a ruling every second round when a player asks me "I stabbed him in the EYE...wouldn't the other goblins maybe run away when they see something like that?" when the answer either is "No, they've seen lots of violence before and aren't afraid" or "Yeah, that's so violent that they are afraid and get away." Both answers are completely valid and I could see it going either way. So it's really a toss up in my mind.
If there was an ability that specifically said "You hit someone in such a way to be SO frightening that others are afraid to approach" I would at least have some in game guidance to help me make my decision. I could say "Well, normally I wouldn't consider stabbing someone in the eye to be brutal enough to scare people, but the game system seems to be in a world where that kind of thing almost never happens so people are really intimidated by it. Ok, I can deal with that as I now know what the baseline assumption is."