Ok, we all know Aizen's zanpakuto's power is complete hypnosis, and the only way to fall victim to it is to look at it. Making Ichigo one of the only people to be unaffected by it. Does this have a limit, there are several who I'm sure he could have used it on numerous times. It seems to have not affected Urahara, Isshin, Yoruichi, and Ichigo. Those four have all been at one point a potential victim of kyoka suigetsu. Yet none of them seem to be phased by it. Also, did Ichigo become too strong for it to effect him, because I'm pretty sure he has glanced at it more than once during his final battle with Aizen.