This is a strong effect, vs many decks it's way better than Loatheb - most of the powerful minions have Battlecry effects. Also shuts down hero cards.
Just look at Highlander decklists - for example my Galakrond Highlander Rogue has 19 minions, of which 15 have Battlecry, plus the Galakrond itself - it's more than half of the deck! Plus all the lackeys from invoking, that's a big deal.
I think this is very very strong and unless Battlecry minions go away from the metagame, this will always find a place in some decks. When N'zoth was available in Standard, deathrattle decks were everywhere. We are now in a Battlecry metagame in which EVERY single deck plays some sort of Battlecry card. Some decks more than others.
Shudderwock Shaman can actually lock out opponents from big turns twice now. Once with this minion and once with Shudderwock. And that means mirror matches too are going to be won on who plays this on the perfect turn to shut out the Shaman opponent.
If nothing else this is meta defining because it affects all decks and might have players adjust how they play or what cards they put into their decks because of it.
A simple example would be, do you play a Battlecry minion with 'Draw a card' or do you NOW play a minion with Deathrattle 'Draw a card' instead? Especially because you don't want to be locked out of a turn where you NEED to draw a card but can't.
This is an exceptional tech card against singleton decks. Every single singleton card is battlecry activated, and this puts the important ones out of reach (Reno, other Reno, Brann, Alexstraza). Shuts down lackeys. Should stop Galakrond too....
The problem with this card is most of the times the opponent can play something else, like a no battlecry minion or a spell and the effect is lost.
Loatheb are played in aggro decks when the only option to the opponent don't lose the game is play a mass removal.
Of course in some matchs, like quest shaman, this card is nuts, the minion destroy armor is very good in very specific situation but in most cases you will prefer play a 5/5 dragon and draw a card instead.
Its a good card but its too situational. Its on the same level as some cards like Destroy your opponents armor and such. In an ideal matchup against Shaman its great but to include it in your deck just in case is meh. For that reason I voted playable. We won't see much of it other than some random decks that like to mess with meta decks for fun. I occasionally use a control warrior with Sabotage guy.