The last time I fought a vampire (in a game. Real life is another story) was the first time I played DnD outside of learning from the box play. For whatever reason, the games I've been in (except those I've run) seem to lack undead - I feel this may be due to the fact most people only use Turn Undead to power Divine Feats.
At any rate, the encounter was memorable - because we ad-hoc'd rules for successfully staking a vampire in the heart. I think instead of making actual attack rolls, we just jabbed out blindly and rolled a d%, with different sections of 100 corresponding to different parts of the anatomy. It became entirely chance - stab the right place (the heart), and it dies. Otherwise, miss entirely.
A lot of fun was had, especially since we hit a few stupid places on the vampire before getting the heart, but today I started thinking - why shouldn't there be rules for hitting a special vital point on a monster?

What I propose therefore is this - every monster has its own weak spots.

All humanoids, for instance, have a heart that is significantly important.
If the AC to hit said humanoid at all is 10 + size + dex + armor + shield + etc...
Then the AC to hit the heart would be 10 + size of heart (tiny) + dex + armor + etc.
Hitting the heart would deal considerable damage - perhaps you might treat hitting the heart as an automatic critical? Or perhaps a straight bonus to damage - perhaps 1 or 2 hp per hit die of the opponent.

Dragons have hearts too. Dragons also breathe fire from notoriously exposed throats. Let's examine a larger dragon.
Dragon AC = 10 + size (abysmal) + natural armor + dex + etc.
Dragon Heart = 10 + size (less abysmal, but still pretty hard to miss) + natural armor + dex + etc.
Dragon Throat = 10 + size (easier than heart) + etc.
Let's say this largish dragon has a massive amount of hit points, and you really need to kill or disable it before it hoses you. It's large and sluggish, so it should be easy to hit, but ouch, would you look at that d12 hit dice. That'll take some whittling. Unfortunately, it's breathing fire (or ice or sonic or psionic or sarcastic damage or whatever) at you every 1d4 rounds, so whittling is an unpalatable thought. Now, you can try and shoot for its heart and try to wound it more severely, as each hit would deal a lot of damage, take it out faster - or perhaps you can try for its throat, and get a special effect, like disabling its breath weapon, which might save you a lot of trouble in the end.

These ideas are sparked by existing mechanics - the weak spots like the heart are like a less random way of critical hitting, and are sort of vaguely covered by the concept of sneak attacking. Instead of hoping for a natural twenty, you're actively trying for an extra powerful hit. Instead of saying that only rogues know where enemies are weak, we say that pretty much anyone (Ooh, or perhaps anyone who succeeds on the relevant knowledge check) knows where to place an especially difficult but potent attack. Taking out the dragon's throat isn't entirely original - DnD is full of ways to indirectly foul up a weapon or tactic. Slow a golem, ice a fire vulnerable monster. Silence a spellcaster, entangle a scout, sunder a melee combatant's weapon. This mechanic simply adds some structure to the fine art of attacking your enemies weaknesses. Some of my other thoughts on weak points include:

  • Giving certain weak points additional defenses: For instance, the heart of a dragon is pretty deep in its chest - Just piercing its scales isn't enough. Perhaps extra natural armor tacked on there to represent the normal flesh around the weak point.
  • Go for the eyes: Everyone's got eyes! And we have rules for blindness!
  • The vampire example - perhaps the weak spot grants no extra weapon damage, but the use of a wooden weapon causes instant death (the stake principle).
  • Balance-wise, how does this work? One possibility is that you'd make weak spots a non-altering part of a monster's stats: A dragon would not change CR or anything just because it has a weak spot, because the idea is that the weak spot is difficult enough to hit that it's a fair exchange - If I didn't bring the right weapon (stake vulnerability) or if the AC of the weak spot's too high, I'll stick to my regular attacks. If I can consistently hit the weak spot, I'm just more quickly dispatching an enemy that I'd be hitting all the time anyway. What this means is that you'd take a monster, look at it's CR and abilities, and assign a weak spot that would represent an option comparable to normally attacking it.
  • Alternatively, you can make these into flaws. For instance, giving a dragon a weak heart (autocrit +1x, auto 4d6 sneak attack damage, whatever) might be a good way of nerfing a monster that you're throwing at your PCs, and maybe we could rip flaws=bonus feats straight off - or even design weak spots so potentially lethal that they represent a NEGATIVE level adjustment.
  • Perhaps this system should replace the critical hits system.

Your own thoughts?