In D&D, warriors to the majority of their deathdealing on their own turns. Which I suppose makes sense from a game mechanics point of view. But, given the existence of attacks of opportunity and what they're meant to represent, why then, are a warrior's attacks far less effective when they are taking advantage of their enemies' foolish moves and/or missteps?
For example, a Barbarian leap attack pounces on a guy, charging and attacking him four times, dealing 2d6+30 damage with each hit. Then, the victim pulls out and drinks a potion, provoking an attack of opportunity from the Barbarian which deals 2d6+20 damage if it hits. Now, in real life, pulling out a potion and drinking it in combat with and in front of a guy with a greatsword will get you killed. But in D&D it results in somewhere roughly between 1/4 and 1/5 the Barbarian's normal damage output.
I've been thinking about this concept ever since I watched Sucker Punch a couple weeks ago, and it just doesn't seem right. In the movie, Baby Doll straight up kills an adult dragon in one attack of opportunity that she got because the dragon made a battlefield misstep. It seems to me that AoOs should be more deadly than regular attacks, because, nearly by definition, they have messed up in a way that opens up their defenses.
So, if we accept the notion that AoOs should be more deadly than regular attacks, rather than a lot less so, how would we go about making that notion a reality? In redesigning D&D, I would rule that all critical hits, always scored on a natural 20, deal double damage, no more, no less, and with such a rule it would be simple and effective to say that AoOs are automatic critical hits.