We've been playing with critical hits for a long time now, my group used the 'score a natural 20 and roll your damage twice, totalling them together' kind of rule for critical hits. I see critical hits as a way to do extra damage in whats an already abstract combat system, not a chance to do extra damage. The chance comes from the attack roll to me. The problem arises when the player, through the good fortune to roll that 20, rolls minimum or near minumum on his damage roll (same goes for my monsters, while not out to kill my player's characters on a malicious whim, I get a bit excited too when I roll that 20 and deflated that it does 2 or 3 points of damage).
We started playing with a new rule and its been working well thus far on both sides of the table:
Critical Hits are scored on a natural 20. The character inflicts his normal damage with all attendant adjustments, plus additional damage equal to half his Strength score (for melee attacks), his Dexterity score (for missile attacks) or his Charisma (for spell attacks, after any saving throws to reduce damage are made).
The simplest rationale for melee and missile attacks are that those 2 ability scores affect those forms of combat already, so we keep them in-house, so to speak. The choice of Charisma is that while Intelligence and Wisdom give you the ability to cast Arcane and Divine magic, the caster's force of will to bend the supernatural to his desires is an aspect of Charisma (how many people have you heard about that don't seem exceptionally beautiful or even good-looking, but you just feel they have some kind of presence). That's my rationale and its worked good so far.
For monsters, I use average stats:
- Smaller than Man-size - Strength 6, Dexterity 14, Charisma 10
- Man-size - Strength 10, Dexterity 10, Charisma 10
- Larger than Man-size - Strength 14, Dexterity 8, Charisma 10
- Giant-size - Strength 18, Dexterity 6, Charisma 10
In the case of giants hurling boulders, those are more like catapults firing at hobbits - here isn't a whole lot of careful aiming to do when hurling a rock larger than your target, your lobbing it in the general vicinity. Charisma is considered average unless you want to roll 3d6 for each spellcaster.