What I love is if you have a bunch of summoned Giants, the AI will of course try to go around them, but if you spread them out, put them in a line but a space between each one, the AI will try to go /through/ them.
What I -hate- is the screwed up initial deployments. For some reason, when you have a lot of units, especially in city defense, a number of your units will be by themselves right next to the enemy, AND several of your units will be plopped on the same square.
I have to learn to move all my heroes out of a city first or they get clobbered.