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I do feel for those where were math deficient though and cannot grasp THAC0. But I am surprised that you or your group would have dificulties with that, but NOT with subtracting 5 off your to-hit rolls and adding 10 onto your damage if wielding a two-handed weapon. That sounds just as hard as saying I have a THAC0 of 18 and rolled a 13 so I hit an AC of 5. Same thing in saying that my AC is being upped by 4 by dropping my attack roll by 4 and having to remember to take 4 off each roll for every round I want to keep it in effect.
As for the d20 mechanic, you completely avoided the fact that the actual DC needed to accomplish your task varies with just about every skill. So you roll and add numbers, if you have to keep looking up what the total number means in terms of success then its not that much faster really.