The other thing that can prevent Mind Control is being mage specific. So for example a Forcemaster can't Mind Control Kralathor because Kralathor is a nature mage only card, and a Forcemaster is not allowed to control it.
I actually did not realize that was the rule. I thought it would only prevent the card from being put in your spell book, but for mind control it would be fair game.
But it makes sense. A necromancer would not be able to enchant-steal Barkskin, for example.