I've found the discussions of
unkillable creatures and
ineffective weapons in DF2010 to be quite interesting. Though there very well may be some bugs making things work differently than Toady intended, I think that a substantial part of it comes from a more realistic model of combat wounds. Many of the wounds that would come from medieval combat would not be instantly lethal.
Slowly bleeding to death from an injury really should be more common than instant death! However, many of the injuries that won't kill you for hours or days should put an immediate end to your effectiveness in combat (severed limbs, head injuries, etc).
The big trouble with the new not instantly lethal wounds is that combatants don't recognize when an enemy is not a significant threat any more. I think that when a creature is unconscious or when several of its body parts have been severed or mangled, it should be considered to be incapacitated by the creatures that are fighting against it. Exhaustion or a stunning blow might temporarily incapacitate an otherwise unhurt creature. When a creature becomes incapacitated the opposing creatures should not fight it as before. Instead they should either ignore the creature (and fight against other more capable enemies) or they should target their attacks more thoughtfully so as to finish the incapacitated foe off quickly.
Ignoring an incapacitated enemy is obviously advantageous to creatures who are fighting against many opponents. It would also make it more likely for injured creatures to survive close fought battles, either because they will leave an enemy to bleed out while they get medical treatment for their own severe wounds, or because a victorious enemy ignored them long enough for them recover (and run away) or be rescued by allies.
Finishing off an incapacitated creature is the other side of the coin. If a fighter doesn't have anything else they urgently need to do, they should try to kill nearby incapacitated enemies as quickly as they can. This sort of attack should be more deadly than fighting an actively resisting enemy! Throats can be slit and spears thrust between ribs. Even against an armored enemy, daggers can be stuck through eye-slits in helms or through under-arm gaps in plate armor perhaps, or an enemy's helmet or other armor could be deliberately ripped off to expose a vulnerable area. Wrestlers could focus on chokes and other potentially lethal techniques (head strikes) rather than twisting a third toe.
Figuring out exactly how such finishing moves should work within the existing combat and wound systems may be a bit complicated, but it might be something that could be folded into a more general "enemy vulnerability knowledge" system (which I think Toady plans to add eventually). Attacks against an incapacitated creature would be much more likely to target a vulnerable spot.
Anyway, what do you think of this idea? I think it will add both balance and realism to combat, as strong, well armored fighters will be able to move from enemy to enemy more quickly, while still being vulnerable to having their throats cut if they eventually get overwhelmed. Hopefully this kind of system would prevent 100+ page combat reports being the norm when armored creatures fight in the arena and stop dwarf mode soldiers from starving to death while trying to finish off a marmot.