I'm interested in the defense of the rule. Do you think it's more realistic to have the AoO coming into a threatened square or retreating from it?
If I'm fighting Mike Tyson, I think it would be hard to even approach him and begin the combat. But, if I were already in the combat, it seems a bit easier to retreat and put distance between us.
If the d20 rules allowed for an AoO as someone ENTERS a threatened space, then fighters would dance around each other, just out of melee range, looking for that particular hole in the foe's defenses in which to strike.
When discussing a game, I find it important for there to be good mechanics. Realism is added on later.
It's also a playability issue... provoking an AoO when entering a threatened square would discourage anybody from initiating melee combat...
If you retreat with your back to your enemy you can't see what he is doing and can't react to it.
i just wanted to drop in and say how i love watching a "Dandu vs WB" thing going on...
PS: AoOs are fine as they are IMO.
The Buddha sees the Buddha nature in others.He gets a kick out of trying to provoke me. But, no AoO's coming from this way. I'm pretty hard to provoke. I don't let these types of things get to me. Life's too short. Besides, I don't respect him. He doesn't want to add to the converstaion in any meaningful way. He'd rather act as a troll and ask inane questions.
I want the rules to incentivize engaging in melee, not sitting off at a distance waiting.
It is not incorrect in the least for my games. I want my players to want to engage with foes on rounds other than the very first round. I want them to want to engage with foes even when luck fails and they lose initiative.But that thinking is somewhat incorrect.
But flat-footed characters and foes with Combat Reflexes . . .Since a flat-footed character cannot take an Attack of Opportunity, having an AoO when entering a threatened space would encourage quick jaunts to engage the enemy.
Is this a measure to punish combat? Is this a measure to punish combat?

Other than that some interesting tactics would come to play...for example at first round stand 10 ft away from the opponent fighter...he will either spend a turn doing 5 ft or will grand you an aoo...or will have to move around you, wasting movement

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.