Priest finally gets to kill 4-drops. I still think this is a pretty good card, even though you have to invest more mana than a plain Assassinate *if you want to kill something big*. In WoToG mage used to run Forbidden Flame as a single-target removal, so priest won't be any different. In some cases this spell is far superior than the mage equivalent (when the minion's attack is lower than its health).
meh, Death and Pain so much better, yes, great thing, you can destroy 4 attack things, but pretty meh. But! Priest always love removals and maybe it will be for some new no minions only clears Priest deck like in League of Explorers.
My first impression is that this card is horrible. It can kill a minion that you badly need to take down but you don't gain any tempo at all. Both Shadow Word: Pain and Shadow Word: Death will be cheaper to use and don't eat all of your mana. In addition to that, I don't like the idea that it can't kill anything that is higher than 10+ attack.