[Cat15]

Other potential issues could arise from things such as elemental absorb, damage retaliation, and effects that proc upon being hit. A Red Letter user could force a Narcus user to heal them, a Brightray Bulwark user could force a Kirosh user to kill themselves attacking them, a Haunted Armor user could force someone to hit them to inflict fear. These are just a few situations out of very many potential unforeseen ones.

About half of the current promoted classes have abilities that make Movement unnecessary in most-to-all situations. In a two-class system, this means that only a handful of viable class combinations won't be able to ignore this system at the cost of a very small amount of FP for no extra Momentum. As such, it seems like this system could far more easily be used to the benefit of those being attacked than the attacker, at least in PvP. Furthermore, this would be an accidental nerf to the minority of class combinations that actually don't have an alternative to Movement, especially if the AoO was a super hit which is unavoidable and ignores defensive effects to mitigate the previously mentioned issues.

On a different note, I also dislike this for the potential negative effects on roleplay. There are IC reasons why one may decide to not attack within a fight. Perhaps the character is refusing to fight back, or has ICly surrendered. Or got unwillingly caught up in a fight in the middle of town or at the arena and is deliberately avoiding any violent action so they don't get arrested by the guards. This could potentially be used to godmod the actions of others in such a situation, since stabbing someone is difficult to ICly explain as something non-violent. "Sorry, Mr. Guard. I tried to avoid the violence, but when he surrendered to you and moved away from me like you ordered, stabbing him in the back was just reflexive."

All of this is, of course, is fairly easy to mitigate if it's a toggle ability.