Easy AI doesn't really seem to do too much.
Normal AI expands well and if you don't jump on it right away, it will maul you. Frankly I see relatively minor differences in the steps between Normal, Hard and Brutal. The step up from Easy to Normal is a big one.
That said, I'm not entirely sure whether an intermediate step between Easy and Normal is necessary. I see Easy as a "testing" AI where I want to set up a particular strategy to see if it works without pressure. Normal and up I see as legitimate pressure testing of an idea.
Maybe there should be a cheating AI above Brutal level. Say, where it gets an extra 20% income? Or two free facs instead of 1?