I got whipped by this earlier in Casual. Seems pretty inoffensive, and I can't complain much since just minutes before I was doing Lady Anacondra + Celestial Alignment shenanigans in Druid just to get a feel for the deck.
Sure it feels a little bad, but it's either a fad that'll go away when they keep losing to Secret Mage and Darkglare, or it does wind up good enough to be tier 2 or tier 1.5, and Warlock just gets another good deck that hard loses to anything with face damage.
While I think minions that make your Hero Immune is silly because it's so limiting of design space in creating stuff like Stealer of Souls, clearly Blizzard doesn't really care about it at all. Personally I think it's not good design. The only card that isn't annoying that makes your Hero Immune is Time Out!, imo. Paly exploits Immune less well than Warlock and Mage can.