I've just recently finished re-reading what is, in my mind at least, one of the greatest fantasy epics ever written: David Eddings' Elenium and the Tamuli. If you haven't read them yet you really, really need to. If those six books don't give you a thousand and one ideas for GMs and PCs alike then you've no business being a fantasy gamer.
That's all beside the point, however. Re-reading those books and then running an installment of our weekly game with my group sort of lit a creative fire in my brain. I kept watching opponents brought to six or seven HP left continue to attack, or a PC struck by a golem's enormous fists get back up and wade into the fray with no ill effect other than being a few hit points poorer than he was a round or two ago. It didn't make sense. In books and movies when a giant smashes you in the chest with his club you break some ribs. When you're slashed across the leg you start to bleed. When someone strikes an armored foe they use the tip of the blade to pierce that armor. When you fall from a 300-foot cliff you don't just take a few d6's of damage and get back up. (Okay, this last example no DM worth his salt would allow...but the core rules as written sure do.) That all said, I started trying to find ways to make weapon damage more realistic without making it too realistic. (You know what I'm talking about, I'm sure; there are limits after all. No one wants to have their PC killed by a single well-aimed thrust.) The other big problem was trying to keep each type of damage balanced with every other. I didn't want to create a situation where one type was clearly preferable to any other, but I wanted to create definite distinctions between the three types of damage. Think about it; in any given session in which the PCs are fighting against creatures without damage reduction, how often does the type of damage their weapon deals ever really come into play in your games? In my experience it almost never comes up...but it should. Slicing someone's cheek with a shortsword and smashing a club into the side of their face are two very different things, yet they both do the same amount of damage.
That said, I sat down to develop a set of rules that my group continues to use. After almost two full months of using the new rules the entire party agrees that they add a gritty, visceral edge to combat. No longer does the party's fighter continue to hack at a single opponent until he's dead; now he fights either to cripple or wound, letting bleeding damage finish off those few opponents he doesn't Power Attack into the next century. Try these out and let me know what you think...it'll be worth your while.
Any slashing attack with deals 5 or more points of damage to a target (after damage reduction) causes 1 point of bleeding damage per round thereafter. During any given round a character/creature may only receive 1 additional point of bleeding damage. Thus, if Tordek deals 15, 9, and 6 points of damage on three consecutive attacks within the same round his target will only receive 1 point of additional bleeding damage at the beginning of the next round. These wounds stack from round to round, so if Tordek then strikes for 10, 9, and 11 points of damage the next round his target will gain an additional 1 point of bleeding damage for a total of 2 points at the beginning of the next round. This damage will continue to be applied against the target until they bind their wounds or receive healing magic. During periods of intense physical activity (such as combat) this damage is applied at the beginning of every round as stated above. If for some reason wounds are not bound or treated after combat is ended they continue to deal 1 point of damage apiece per minute.
Obviously creatures with no blood (such as constructs, oozes, and the undead) are immune to this effect.
Binding wounds caused by slashing damage is a full-round action which provokes attacks of opportunity against both the person doing the binding and the person being treated. Binding the wounds successfully requires a Heal check with a DC of 10. For every point over 10 which the person binding the wounds rolls, an additional point of bleeding damage is negated. (ex: Rolling a 10 stops 1 point of damage, 11 stops 2, 12 stops 3, etc.) A person may bind their own wounds with a -2 circumstance penalty. Any Cure or Heal effect instantly stops all bleeding in addition to its normal effects. (Cure Minor Wounds is a perfect spell for controlling bleeding during combat when binding wounds would be difficult.)
Piercing attacks are capable of effectively lowering the AC of a target. Piercing attacks made against a target with an armor or natural armor bonus automatically treat the target's AC as being 1 point lower. (This is effectively a bonus to hit.) An attacker wielding a piercing weapon may choose to take a circumstance penalty on his damage roll and add a bonus to his attack roll equal to the damage penalty. This bonus to hit may never exceed the total of the target's armor and natural armor modifiers. If the damage penalty taken would reduce the attack's damage to zero then no damage is dealt to the target, even on a successful strike. (This represents the armor dissipating the full force of the blow.) For example, Lidda is facing off against a human fighter wearing a chain shirt (+3 armor bonus). Lidda is wielding a dagger and receives a +1 modifier to damage rolls due to a high strength score. On her attack Lidda decides to take a 2-point circumstance penalty on her damage roll and adds 3 points to her attack roll (+1 for piercing, +2 for her circumstance penalty). She hits! Normally, her damage would've been 1d4+1 but her circumstance penalty of 2 points reduces that to 1d4-1. She rolls a 4 on the d4, subtracts the penalty, and does 3 points of damage to her opponent.
Also, any piercing attack which deals 10 or more points of damage in a single blow (after damage reduction) causes a bleeding wound exactly like that caused by a slashing weapon.
Any time a character takes bludgeoning damage there is a chance that they will suffer broken bones in addition to any hit point damage. Broken bones deal 1d2 points of temporary Constitution damage and, depending on what bone is broken, can cause a number of other negative side effects. The chance that a bludgeoning attack will break a bone is equal to the damage dealt. Thus, an attack which deals 6 points of bludgeoning damage (after damage reduction) has a 6% chance of breaking a bone. Critical hits have a greater chance of breaking a bone and thus multiply damage dealt by 3 to determine the percentage. A critical hit which deals 12 points of damage has a 36% chance of breaking one of the target's bones. To determine if a bone is broken by an attack, roll d%. Any result under the break chance of a given attack breaks a bone. There are charts in the DMG that list suggestions for penalties due to damage to a specific area and I recommend using them to determine what (if any) additional penalties a broken bone incurs upon the target.
A character may ignore any penalties from broken bones (except the constitution loss) for 1 round by passing a Fortitude save with a DC equal to 15+the number of broken bones they have. This saving throw is a free action. If this save is successful they may act normally for the duration of the round, but receive 1d3 points of non-lethal damage per broken bone at the end of their turn.
A given individual may receive more than one broken bone from bludgeoning damage in a single round.
Creatures with no bones to break (like oozes, elementals, and constructs) are not subject to this rule.
After having playtested this rule extensively I'd like to recommend that you apply the rules as normal for a character up until they suffer five broken bones, after which remove the Constitution point loss but continue to apply all the other penalties from broken bones. I'm suggesting this because a lot of bones are going to be broken during a fight with giants or golems and a 5 to 10 point Constitution penalty plus lots of negative modifiers to combat rolls is a big enough disadvantage to work through. If you do, however, continue to use the rule as written don't be surprised if your PCs start to completely avoid fights with creatures with strong slam attacks which, while being very realistic, isn't usually much fun for the players. Alternatively, you could allow a character to make a reflex save with a DC equal to the break chance of the blow in order to avoid the broken bone (but not the hit point damage!).
Falls from a great height (and even a not-so-great height) can be horrifically hard on a body. In addition to regular hit point damage received from a fall, there is a percentage chance that one or more bones will be broken on impact. The percentage chance is equal to the total number of feet fallen in an uncontrolled manner minus 10. (Thus distance fallen while Feather Fall or a monk's Slow Fall ability are in effect do not count.) Reflex saves or other conditions that effectively lower the distance fallen for determining hit point damage also lower the distance fallen for calculating the chances of breaking a bone. Since a fall can cause more than a single broken bone, after each roll of the d%, subtract 20 from the total number of feet fallen. If the number is still greater than 0 reroll the d% using the new number to determine if another bone is broken. Continue this way until the number of feet fallen is reduced to 0 or less. ex: Devis falls 130 feet from the top of a cliff. He has no way to slow or control his fall, so he will use the full 130 feet to determine both hit point damage and the chance of breaking a bone. For hit point damage Devis will take 1d6 non-lethal and 12d6 lethal damage. He will have to roll d% above the following numbers to avoid breaking bones: 120, 100, 80, 60, 40, and 20. Before he even begins to roll the first two chances are 100% or greater so he automatically breaks two bones. After that he must make four more rolls to determine whether or not he broke any more.
Each broken bone deals 1d2 points of Constitution damage and may cause any number of additional negative side effects.
Although falling damage is technically bludgeoning damage DO NOT apply the rules for bludgeoning damage to damage taken from a fall. They're lethal enough as is.
Mending Broken Bones
A heal check must be made to set each of the broken bones a given character has. Each heal check takes one full minute and provokes attacks of opportunity upon both the person making the check and the one being treated. A character may attempt to set his or her own broken bones at a –5 penalty to the heal check. All such healing checks are made at a DC of 15. Success on the roll deals 1d4 points of non-lethal damage to the injured character, but negates all penalties from the broken bone other than the Constitution damage. Failure on this check results in the bone not being set and the injured character suffering 1d6 points of non-lethal damage. Once a bone has been set using this method it heals at the same rate that the Constitution damage resulting from its being broken does.
Alternatively, and perhaps more commonly for PCs, healing spells can be used to repair broken bones immediately. Lesser Restoration and/or Cure Serious Wounds will automatically set (but not bind!) all broken bones in the recipient. Until the bones are splinted or otherwise bound the character may not move that part of his body without un-setting the bone in question. (Applying a splint without the need to set the bone requires a Heal check with a DC of 10. This check can be made by a PC upon themself, but incurs a –2 penalty to the roll.) Restoration and/or Cure Critical Wounds will set all broken bones in the recipient and heal one-half of the Constitution damage dealt by their being broken. It can ONLY heal Constitution damage done by broken bones. Once again, the recipient will need to have his bones splinted or bound in some way. Any Heal or Cure spell of a more powerful nature than the spells listed here completely heal any broken bones the target has suffered, including all Constitution damage dealt by the breaking of the bones.
Note that effects at the same power level do not stack. For example, two castings of Restoration or one of Restoration and one of Cure Critical Wounds still only heals half of the Constitution damage dealt by broken bones.