At first glance, I would have thought the warlock hero power required a complete mechanical overhaul. However, the unfortunate truth is that the warlock class cards are balanced around the assumption that the warlock has a way to draw cards. Many class cards force the warlock to discard, and not one enables him to draw cards, so a complete redesign of the hero power is out of the question because if the warlock cannot draw cards with his hero power he will quickly become the worst class in the game
So working on the assumption that the warlock hero power is too powerful and it must be a hero power that draws cards, the only option remaining is to increase the cost of using the ability. Either by increasing the self damage, the mana cost, or attaching some other negative effect to it. Unfortunately though, because you cannot have half a life or half a mana, the only options are costing the mana cost at three, (making it largely unwiedly) or burning the player for 3 life (Just making the deck more suicidal and not really solving the issue at hand)
The only remaining option is to attach some other side effect to the ability, but I have been unable to come up with something that doesn't feel too heavy handed.
Do you have any ideas?