In earlier incarnations of D&D we had useful, crunchy morale rules. The 1st edition Advanced rules were messy, like most 1st edition subsystems. But the 2nd ed morale rules and the Basic/Expert version were very useful mechanics. These morale rules combined with my groups tendency to not use a tactical display allowed for a lot fights to end with 'and the rest of orcs run away, never to be seen again.'
I'm not a huge proponent of 'realism' in my dungeoning and dragoning, but sometimes I think the tendency for D&D combats to turn into total slaughters is a tad bit too video gamey for my tastes. As the title goes of the post goes, sometimes fights just stop. I don't spend a lot of time studying real violence, but I've seen examples of brawls where the atagonists trade some punches, scuffle a bit, and then just quit fighting. Whatever psychological energies led the parties to blows had been expended, and the tusslers just stopped. And whole wargames (I'm thinking particularly of Sniper!) have been written about the skirmishes in war where two forces meet unexpectedly, trade some gunfire, and then one or both sides just leave.
Fiction is chock full of these sorts of conflicts as well. Comics are chock full of stories where the first act involves an unsatisfactorily concluded fight with the villain or his henchmen. Or think about the light sabre duels in Star Wars. Qui Gon's first encounter with Darth Maul and Luke's with Vader were both inconclusive. Well, Luke got a severe ass-beating, but I think you see my point. No one died and, more importantly, the narrative tension between the combatants was not released. Sometimes this combatus interruptus is caused by one of the parties ending the combat intentionally, other times circumstances align to make continuing the fight no longer viable.
So I think I've got two intersecting issues here. First, from a strict realism argument, more badguys should run away or surrender than I've normally seen happen. A simple morale rule would help here. Something like the old Basic rule, where every critter had a morale stat and you checked at first blood and 50% casualties, but without inventing a whole new stat. Maybe a DC 15 or 20 Will save?
The second issue is basically looking for a way to build dramatic tension by delaying gratification. We all know that the villain getting away is set-up designed to make finally nailing the bastard all the more savory. Yet played by the book D&D (and many other RPGs) either makes it hard for the villain to get away or else makes it really, really lame, such as the kind of game where every master villain always teleports out of danger.
I've seen a couple mechanical solutions to this problem over the years, but not many. The original Marvel Super Heroes game allowed master villains to spend some Karma (that game's version of Luck Dice or Fate Points or whatever) to get out of jail free. As long as the villain had the unspent Karma, he was pretty much immune to capture. I thought that was a pretty slick design decision, especially in the comic book superhero genre, where recurring villains are the standard. Eden's Buffy the Vampire Slayer rpg gives players more Luck Points (or whatever they're called) whenever the GM pulls stuff like blatantly escaping the villain by fiat. Both of these a pretty good solutions, I think. Especially if you allow players to spend their own Nifty Points to prevent the bad guy from getting away in the next encounter. S. John Ross' Risus neatly solves the problem by making the fate of the defeated the choice of the victor. Given the general comedic bent of the game, most foes end up humiliated rather than destroyed.
But here's my own idea for villain escapes: time limits on preliminary encounters. Here are the examples I came up with when I discussed this problem with Stuart. The basic idea is that fights have a limited duration, after which the GM (or a player, if you're playing that kind of game) narrates some sort of ending.
Okay folks, you have 2d4 rounds to play out this prison riot before the guards totally shut the fight down. Go!
Here's your challenge people: If you don't bring down the big bad in 5 rounds he is totally going to get away.
Time's up? Well, dang. The house in on fire, right? Howzabout falling timbers separates us from the baddies?
Do you see the advantages to this method? Without any real mechanical alteration you make it easier to build up to that final confrontation with the Big Bad via a series of brief and unsatisfying encounters. And you can totally pull off cinematic stuff like the fight ending because the ice over the river breaks up or the volcano erupts or whatever. I'm not saying every encounter needs boundary conditions and a scenery destroying ending, but used judiciously this technique could really punch up a game. As much as I like the overall concept, I wouldn't pull this crap every session because it would be a little too railroady for my tastes. And none of my NPCs are so precious that every one of them needs a chance to get away. I build those creeps specifically for the PCs to beat down.