As Jowy says, I think the Emperor will be most problematic in combo decks. Turn 7 Force/Nature. 7-mana Fireball/Fireball/Frostbolt/Ice Lance/Ice Lance. Any number of Rogue shenanigans - I mean, if Rogue could work miracles with concealed Gadgetzan, what could she do with a concealed Emperor? In fact, if you can't remove Emperor immediately, it's probably GG. The card won't help much against aggro, true, but that's another way of saying that aggro must become even faster, because it can't compete with this even in the midgame. (It's much better against aggro than the likes of Sneed's or Ysera.) And in control matchups, the first to play the Emperor probably wins (like Alexstrasza in the Freeze Mage mirror).
I'm not sure Sylvanus or Cairne are in the same league; they can be silenced, in which case they're strictly worse than Boulderfist Ogre. GvG even introduced an arguably better Cairne (Piloted Sky Golem) and it hasn't seen all that much play. If the card is to be nerfed, I think they'll make it a one-time Battlecry - which would still be very good.
I'm not sure Sylvanus or Cairne are in the same league; they can be silenced, in which case they're strictly worse than Boulderfist Ogre. GvG even introduced an arguably better Cairne (Piloted Sky Golem) and it hasn't seen all that much play. If the card is to be nerfed, I think they'll make it a one-time Battlecry - which would still be very good.