Just tells you what state the game is in when a broken hero power like that is a liability when most decks don't care that you have a 10 cards in hand, the health loss just means they get to kill you faster.I still dont know what were they thinking while designing warlock hero power, suck powerful effect shouldnt be granted for free to a hero. Cards that let do that on MTG multiple times are banned from tournament play...
Handlock has to use ridiculous comeback mechanics like Moltens/Shadow Flame/Jaraxxus/massive heals to stand a chance against the field.
That said there will be some meta where Warlock will be the best. If they keep introducing good ways to counter aggression and heal face then Warlock will eventually be the best class. Until stuff like 14-15 damage from hand combos exist in the game, Warlock will not be on top.