Aizen has a common flaw of being over confident and cocky once in a battle he has shown this with his fight with Ichigo Kurosaki.
Aizen may major power set is because of the Hogyoku which causes his transformations without it he mainly uses his intelligence along with his powerful Zanpakuto to win a battle.
Aizen Kyoka Suigetsu ability does not work on the blind or those who have not seen its release making his attack useless.
Aizen Zanpakuto has a Near Flawless Illusions meaning beings such as Yhwach.
Aizen when the Reiatsu of Kyoka Suigetsu release will give away its essence to only beings who are very keen enough to sense its actual presence and to get past its near flawless illusions created by its complete hypnosis.
Another weakness of aizen very own zanpakuto is to touch the blade before its activated as shown with Gin Ichimaru during his betray.