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.
Well it is a very versatile card, but is that versatility strong enough to make it a good card?
It is a very slow card if you want to kill a big minion, you can kill an 8 mana minion for 3 mana with Shadow Word: Death. You gain 5 mana of tempo. You will need to spend around 8 mana to kill and 8 mana minion with this. You gain no tempo and it is a reactive card, so its absolutely not good.
But if we compare it to Shadow Word: Pain, it might be actually better. It can't target minion, but I guess it will always kill the highest attack minion, which in most cases that's exactly what you want to do. For 3 mana you can do what SWP does for 2, 1 extra mana shouldn't make a big difference, versatility will negate that. And on the upside it can kill Hench Clan Thug, it can kill Twilight Drake it can kill Tundra Rhino for 2 mana etc.
So overall I think this card will replace Shadow Word: Pain in priest decks due to its strong versatility.