Voidcaller seems like a pretty damn good card, though. Being able to pull in something like a Pit Lord, Felguard, Doomguard, or Dread Infernal without the associated mana cost or battlecry cost seems pretty huge. Hell, even grabbing Jaraxxsus and putting it into the field would be pretty crazy. Having a 3/15 body on the board is freaking crazy. Only a priest could really deal with that.
I like demons more than the meta does, but that's an overstatement. Nearly every class can deal with a 3/15 minion with just one card or a very common combo. Mage has Polymorph, Druid Naturalise, Hunter has the Hunter's Mark combo or Deadly Shot, Paladin has the Equality combo, Rogue has Assassinate, Warrior Execute, Shaman has Hex.
I guess the only class in trouble would actually be Warlock themselves because no one runs Sacrificial Pact or Corruption these days.
Of course, Jaraxxus, Illidan or Dread Infernal would be a hit, but for every one of those you could also miss with a Voidwalker, Blood Imp. Void Terror or Flame Imp. I think Voidcaller is better than the similar cards Sense Demons or Bane of Doom, so I like it.