Hearthpwn

It's because they have a broken core card set (basic + classic). There are essentially no core cards in that set, there are no build up cards and you could argue there is no identity to the class. However, there are loads of finishers and super efficient burst. That means that in order for Shaman to work, you need to introduce in large numbers very strong core and build up cards in expansion sets. Once there's enough of those, Shaman suddenly becomes broken because of all the available burst. It's all or nothing with this class.