I find the way AI generals charge pointlessly into your infantry usually phalanxes for some daft reason is perhaps the most obvious and consistant stupidity on part of the AI, which is annoying especially since once the general is dead the entire battle is practically over as enemy troops flee pretty soon after their general is dead.

It didnt do it so much on Medieval but on Rome for some reason the AI is 10 times dumber, I just hope on the expansion they remedy this somehow.