It would be great if D:OS had ways of defeating opponents without killing them.
First of all, it's not realistic that every single enemy will fight to the death. Animals, even the hungry and aggressive ones, might try to flee when it becomes obvious they're losing. More intelligent opponents might surrender in order to save their lives.
I think you should get XP for every defeated opponent, including those who successfully flee or surrender. In this case, it might be necessary to tag those opponents, so you cannot generate an infinite amount of XP by attacking fleeing opponents over and over.
Of course, you'd miss out on the loot of fleeing enemies. Those who surrender could offer you all their belongings in order to get away, though. Some might even offer additional loot, like telling you about a secret stash of items if you let them live.
It would be really cool if your reputation influenced the chance of surrender: As long as you're an honorable person who is known to show mercy, enemies are more likely to surrender. If you kill people who have surrendered, future opponents will be less likely to make the same mistake, and their resolve might even allow them to put up more of a fight.
Of course, this shouldn't happen all of the time. Mindless creatures like some undead should never run away or surrender. Fanatics and soldiers who know that their superiors will kill them if they lose the fight should also have a very low chance of giving up. Untrained citizens, on the other hand, should be quite likely to beg for their lives instead of fighting to the death. It would also add a lot to the credibility of the game world if a heavily wounded opponent became very likely to surrender after you've obliterated the rest of the group without taking much damage yourself.
I'd also like to have a way of disabling opponents even if they don't surrender. You could knock them unconscious or wound them in a way that doesn't allow them to fight on. Why would you want to do that? You might be convinced that your opponent is out of his/her mind (e.g. when possessed or mind-controlled) or you might want to give him/her a chance of seeing that what he/she's been doing is wrong. That might lead to the NPC remaining in the same area, but becoming non-hostile afterwards. It could also leave him/her totally unimpressed, or he/she might even try to track you down in order to avenge the defeat by "purging" this "dishonour".
