Every once in a while I re-read the combat rules in the Player's Handbook just to keep them fresh and sharp in my mind, and while I was doing that most recently the part about knocking an enemy unconscious at 0 HP instead of killing it struck me.
Now this is the sort of thing that makes some people laugh about how cartoonishly unrealistic the game is: "What, so I put an arrow through the goblin's heart and then decide to take him alive?" Of course that's not what's happening. What is happening is that the game is acknowledging that defeating an enemy can take more forms than putting an arrow through someone's heart.
So this got me thinking about all the ways in which a person can be defeated in real life. All 0 HP means mechanically for NPCs is that they are out of the fight. What actually happens is largely fluff, with the possibly significant distinction that they may either be alive or dead.
If you assume that death is the most common or default outcome, there's not much reason to dwell on what exactly happens when an enemy is not slain. But for those who are playing less bloodthirsty heroes, or running less bloodthirsty games, here are some other possible ways to interpret the act of reducing someone to 0 HP.
This works best when players know when they're likely to inflict a "death blow"... I like to make the minions in any fight clear and update players as an enemy nears defeat. If you don't do those things then these options will be less relevant to your games.
Assuming that players do know when their next shot is likely to render an enemy unfit for continued duty, then they can describe their intentions at the same time they attempt the attack. Alternately they can amend themselves when they know the enemy has been defeated by their hand, but knowing up front what they intend can prevent micro-retcons if you're narrating things as they happen.
Some of the alternatives presented below might sound similar to things that can be achieved using specific Martial Attack powers, usually with more limited and narrowly defined effects. There is no conflict between the existence of such powers and the options presented below. The powers are about how one deals with an opponent who is still an active threat. The options below are about how one deals with a defeated opponent. They are as different in game terms as a fully-stat'd monster is from a small child in a village.
Disarming An Opponent
Some players grumble that 4E offers no disarm mechanism and is, in many ways, not at all built to deal with a disarmed opponent. In fact, for a creature that relies on weapons, it may be best to treat it as being out of the fight completely when it's been disarmed.
If disarmed is the same thing as defeated, then the state of being defeated can be used to represent being disarmed.
Imagine your Ranger is trading arrows with a group of minion archers. If the player says, "I'm going to splinter its bow with my next shot.", there are a few ways you can handle it: you can invent some kind of called shot penalty or use the rules for attacking an inanimate object, and then if it succeeds keep the minion in the fight but unable to make its ranged attack... or you can resolve it like a normal attack, with a hit meaning the Ranger's arrow has destroyed the minion's bow.
The minion then either surrenders or runs off. In either case the PCs can expend further attacks on it, but it should be made clear to them that at this point it's a non-combatant.
What about non-minions? Well, we must assume that an enemy archer will no more stand still holding its bow out as a target for enemies to snipe at than it would do so with its own body. Therefore the ability of the PCs to inflict a devastating blow on its weapon, the thing that keeps it in the fight, is the same as their ability to inflict a deadly blow on it: it depends on first whittling down its HP.
(As a sidebar, if I were to write a mechanic for disarming opponents before they're defeated, I would give it similar restrictions as Intimidate has... requiring a bloodied target and rolling against Reflexes with a hefty bonus, for instance... since it would have the same effect: "defeating" an opponent without removing all of their HP.)
Forcing A Surrender
Above I mention that disarming an opponent might force them to surrender, but so might getting the tip of a blade pointed at an opponent's heart or throat, or using an arrow to part the target's hair or put a hole in its hat, or otherwise demonstrating your ability to end the enemy's life at your leisure.
This is a similar idea to knocking an opponent unconscious: you could have made a killing blow but you didn't, and the target knows it.
This doesn't step on the Intimidate skill's toes because it's using a different approach: with Intimidate, it doesn't actually matter if you can carry out your threat or not because you're relying on your strength of personality and/or skill at intimidating to make your opponent believe you. When you force surrender with an attack, force of personality doesn't apply: you demonstrate that the opponent cannot hope to win.
Capturing An Opponent
This works best if the opponent in question is the last one left standing, because in in its simplest, least environmental-dependent form it would be similar to forcing a surrender but you'd keep doing it: keep your next arrow nocked and trained on the target, keep your blade to the target's throat.
If your character has ammunition or weapons to spare, it may also be possible to trap an opponent by pinning them to a wall, tree, or other object or surface.
Depending on the environment, other possibilities may exist: an enemy could be knocked into a basket or shoved into a wardrobe, tripped into some rigging, etc. All of these things are things that could possibly done with a Bull Rush or skill check or page 42 stunt of some kind anyway, but if you take "0 HP" to simply mean "defeated", then there's no reason you couldn't let a player take that defeat to mean forcing the enemy into a position where it can be inconvenienced in one of these fashions. It usually shouldn't matter if it would realistically hold them for long or not... fights tend to take very little "game time" to resolve.
In the middle of a fight, of course, the default non-lethal method of defeating an enemy can be the best way to take a prisoner: knock them out. This can be after they've been disarmed or forced to surrender, for added color.
Debilitating Blows
This option may be less bloodthirsty than killing but may appeal to players who think of their characters as crueler or more pragmatic: an arrow to the leg, a bleeding wound that will require immediate attention... debilitating blows are ways of keeping an opponent from being a danger or a flight risk.
Knocking Over
This choice does not necessarily make for a less lethal alternative to a death blow, but it can make for a dramatic or interesting one that ties the action to the environment. A defeated opponent may be knocked over a ship's railing or off a balcony, off of a ledge, and so on.
In roleplaying games that follow a heavy paradigm of "kill the monsters and take their loot", this can lead to suboptimal outcomes, but with 4E discouraging the notion that heroic adventurers strip the corpses of their enemies to sell at the next town there's rarely a downside to allowing for this classic exit for enemies.
The Dramatic Compact: Playing Things Straight
Encouraging this kind of thinking can add a lot of color to combats, as well as get players thinking more about the non-death option. But these things only work when you the DM not only allow players to do them, but you follow through on them. Simulationist DMs (or gameist ones who don't like "losing") might be thinking, "Okay, but the monster is still there. It's not dead. It's not unconscious. Why can't I just attack with it?"
You can't attack with it because it's been defeated. Defeated is a notional status that, like "minion" or "elite", exists only as a storytelling shorthand for more complicated conditions within the game world. Mechanically, a monster that's defeated... who surrenders, or is disarmed or entrapped by a final strike... is a monster at 0 HP and should be treated as such.
A monster with HP who gets knocked into some netting is still in the fight and can struggle to get free. A monster with no HP who's knocked in the same netting is out of the fight (because the fight has gone out of it) and can't. It's like the difference between being a character that's simply knocked prone, and one that is prone because it's unconscious because it was reduced to 0 HP.
Things that allow a monster at 0 HP to come back and make another desperate attempt to bring the PCs down are very rare in the game. Situations where a defeated monster takes another shot at the PCs should be equally rare. If you're the kind of person who would find themselves thinking, "But logically, in this situation, nothing would stop this monster from drawing its dagger and continuing the fight..." then these ideas probably aren't for you.
If you can handle a little more abstraction in your simulation, though, this might make it easier to draw a fight to a conclusion that is interesting and satisfying as it is decisive.
