This... isn't good? The ability to get spells is one of the reasons Thoughtsteal is a good card. I don't think I'd ever use this. For the chance to not get a spell you have to pay 0.5 mana more for a card?
Decent in a control deck. If you can copy and kill your opponent's heavy legendary, you will be ahead in hand value. The problem is that you can just put a heavy legendary in your deck to have the same effect. You lose some versatility but is more consistent. Unless you want to copy a class legendary like Tirion Fordring. Meh. It's decent. Maybe some Control Priest will run it.
So, basically you pay an extra 2 mana to play any minion that they have on the board? Extra choice on "what minion do I need" could be good. Say choosing a Sludge Belcher or a Healbot... or choosing Dr. Boom or Thaurissan.