Rarity, #: Uncommon, #211
[...] The release of Tarkir: Dragonstorm in April had a dramatic effect on the Standard metagame, propelling Izzet Prowess to the top of the metagame on the wings of Cori-Steel Cutter." Klomparens says that this deck accounted for 40 percent of the Standard metagame at a recent event, so it's understandable why these bans are being put in place
Exiling creatures to make 4/4 zombies is already strong, but it gets better when combined with mill engines like Lord Xander, the Collector or Phenax, God of Deception
The deck plays plenty of ways to sacrifice for value as well with cards like Priest of Forgotten Gods and Riveteers Ascendancy
Any enters abilities of the copied creature will trigger when the token enters the battlefield
A 5/4 for 6 generic mana is actually fair, but that's no longer good enough for players, even in Limited