We have a dozen different powers that "stun" a foe. Couldn't there have just been one that let the PC define how he did it?
In some cases there is arguably needless duplication (eg there are two powers, I think - an Avenger one and a Swordmage one - which are identical to, or very nearly identical to, the fighter power Footwork Lure). But in general, the gameplay reason for different powers is the same as the reason for different classes: to ensure diversity of approaches to the game.
(There is also the marketing reason noted by [MENTION=205]TwoSix[/MENTION], and I think also [MENTION=6688937]Ratskinner[/MENTION] further upthread.)
What you just described is the textbook "reaction" power: a foe does something and the player/character responds.
"Reactions" and "actions" are metagame notions - devices for regulating turns in the action economy of the game. They don't correspond, except very loosely, to distinct categories of events in the gameworld. [MENTION=27160]Balesir[/MENTION] has already made this point in relation to melee exchanges. It can also be applied to OAs: one natural way to understand OAs vs archers and casters is that, when fighting a foe who isn't fighting or parrying back, you get more chances to hurt them. Instead of upping the rate of attacks, though (which can happen in AD&D when very fast weapons are used vs very ponderous weapons), we grant opportunity attacks.
In the actual gameworld, the fighter is just attacking - there is no ingame distinction between the action, the immediate action and the opportunity action.
Technically, the opponents can't do anything once they get in range until their turn.
This seems like another example of reading the action economy rules back into the gameworld in an unhelpful, stop-motion way. Pleading is an "action" in the ordinary English sense, but I don't think it's an "action" in the action economy sense. No game rule is violated by saying that the victims of CaGI have hurled themselves at the feet of the fighter and are begging for mercy.
It's the idea of archers or spellcasters charging heedlessly into melee that really got old fast.
So you can't conceive of a character motivation or situation that would prevent a rational being from charging forward and engaging in melee?
CaGI only works if (i) the target is within 3 sq of the fighter, and (ii) can end his/her move adjacent to the fighter. The distance, therefore between the fighter and target may be as little as 10' (if we assume that each is at the nearest edge of his/her square), and between the target and the fighter's weapon the distance might be as little as half that (depending on reach).
In other words, those who are getting drawn in by CaGI are already in fighting distance of the fighter. (In AD&D, striking distance was 10'. It is only 3E and 4e that reduce default reach to 5'.) They are hardly "charging heedless into melee". At least, there is no need at all to narrate it that way.
It forces the orcs to charge. It takes the agency away from the character's controller (the DM in this case). It doesn't require the foe to enter to activate, it MAKES THEM MOVE THERE to do it. You can move 6 squares, activate CaGI, and force a foe that was 40 feat away to now engage the fighter in melee
As I already posted, I don't feel the outrage. Since OD&D players have had the ability to impose the "dead" status on those orcs. Why is it more outrageous to impose the "moved from there to here" status?
there are certain things that should be the pervue of magic: controlling other creature's minds is one of them.
If you're saying that the fighter is doing it with something beyond mortal ken
I have no powers beyond mortal ken. Yet I control others' minds ever day. I extend my hand to shake theirs, thereby prompting them to move their arms. I speak words to them in languages they understand, thereby causing them to have thoughts. Etc etc. In the real world, a person's exercise of agency is constantly shaped by the actions of others. (Those who are enlightened in the technical Buddhist sense are arguably exceptions. I don't think there are many such people around, however. They are certainly not the norm.)
When it comes to gameplay, the question becomes "Who gets to decide what impact the agency of PC X has on the agency of PC Y?"
Should it not be the pervue of the DM to decide if the orcs want to flip out and charge? Isn't that what the DM is there for?
CaGI breaks the game assumptions by overriding the DMs control of his NPCs
There is no such assumption made by the game. It doesn't assume that only the GM can decide when an NPC is dead. Nor does it assume that only the GM can decide when an NPC's location changes.
B/X and 1st ed AD&D had both reaction and morale rules. In OA, samuarai and kensai could cause fear. 3E has Diplomacy rules. 4e has social skill challenges. I don't know 2nd ed AD&D well enough, but no other edition has assumed that all decisions about NPC behaviour are matters of GM fiat. Nor has any other edition of D&D taken the view that things outside GM control must depend on dice rolls (eg the classic Sleep spell; magic missile; fireball vs an opponent with only 1 or 2 hp; etc). CaGI it combines past elements of D&D in a new way - a player determines certain NPC actions without the intermediation of a dice roll - but none of the elements is new.