Well, really that it's not possible to be omnipotent and perfectly good and create evil. The last two Mill sees as mutually exclusive - one cannot be "perfectly good" and create evil. Something has to give, according to his reasoning, and so he chooses to abandon omnipotence as a way of avoiding the necessity of assigning responsibility for evil to God.
The presumptions in that statement are myriad, e.g. good v evil, their origins, that omnipotence and evil are mutually exclusive, that mortal minds can comprehend the mind of God, the significance of events within space/time to events outside space/time, etc.
Well, one at a time. If objective morality exists, evil must also objectively exist. And, in fact, we can observe evil all around us every day just by turning on the evening news. As for the origins of good and evil, an omnipotent God, creator of everything, must have created good and evil - surely you're not going to argue that good and evil exist independently of God? ;)
As for the last two, whether mortal minds can comprehend the mind of God is not strictly relevant to Mill's argument - Mill observes God's handiwork, His creation, and reasons backwards to determine what sort of being would create evil. Answer: not a being that is "perfectly good". But this is not acceptable to Mill, or, he suggests, to most people, and so he posits instead that God is in fact perfectly good, but not omnipotent, and so could not necessarily have prevented the creation of evil - as opposed to an omnipotent God, who we have little choice but to accept created evil Himself. And by the same token, its significance in some larger-than-life sense is not strictly relevant either - the mere fact that evil exists is where the problem comes from, in Mill's thinking.
Evidently Mill has a problem with this view.