An 8/8 taunt for 8 isn't anything special, I think. In most matches, he won't live beyond a turn, so you're just getting one extra card. Ysera sees no play, and while she doesn't have Frostmourne, all of her spells are generally playable, while Lich King's spells range from unplayable to mediocre to Frostmourne, depending on your deck and situation. In 7/8 plays you won't get Frostmourne, and if you look at all the other stuff you can get I think the card becomes questionable.
I think it will have its place in a few decks, but people are going nuts for something really inconsistent and risky (tempo loss). Like, the only deck I would run this in over Ragnaros in Wild is Quest Warrior, and a Taunt Druid or Big Body Druid deck if that ever becomes a thing.