The 'whole play the cards your opponent played last turn' could be good, but assuming targets will be chosen randomly, like Tess Greymane, I don't see this being very playable. Your ultimately spending 8 mana for an 8/8 and to do the same thing your opponent did, but with much less control, and it's just as likely to make things worse and lose you the game.
Plus you don't get the value of things like Battlecries when your opponent plays a battlecry minion.
I think Murozond, The Infinite would be INFINITELY more playable, if instead it added copies of the cards played by your opponent last turn to your hand, but they cost (0). That way, not only do you have control over the outcome and order, you actually have the flavor of Murozond going back in time and trying to change the past.
I don't like this card that much. At first glance it looked super cool, but than, this card looks so passive. I think usually you want to get ahead of your opponent and playing his card +8/8 minion doesn't look like the most powerful turn. Especialy taking it consideration (in removed already video) card did not trigger battlecry efect from yogg.
First of all this card requires you to wait for good opportunity, and this is not something you want to do with reactive card. on top of that you loose some of the effect, like battlecries. Sure you can play 18 mana value for only 8 mana (if your opponent played 10 mana card before you), but usually it doesn't answer board state.
Imagine opponet plays deathwing to threaten lethal, playing 8/8 +12/12 means nothing if you are dead, you need proactive answer not passive effect. You can prise to RNGods for luck with coping firbals or pyros, but usually its not a way constantly to win games.
Best i can imagine is copying some strong rush/charge minion, or specific spells like flamestrike, secrets, blizzard, etc., but how often can you wait for such an opportunity? You can wait for galakrond, but wouldn't you rather play your own game?
Good points. I think the best case scenario for this is Priest first using his control tools to clear the board and preserve health into the late game, then on turn 8+ opponent make a strong move to retake the board and you respond with this. Regardless, I'm glad they've gone back to pushing the Priest archetype "use your own deck against you" as I think that's the funnest one for Priest (opposite of boring Rez Priest) and it makes for some of the most interesting and interactive games in Hearthstone.