Losing health equal to the cost of the card drawn (Warlock Hero Power) is a terrible decision. That's not a nerf. That's gutting the class.
I'm not willing to go quite so far as to say that the Hero Power is overpowered but it is, clearly, the best. The Hero Power is the reason Zoo works. That's why other classes don't run Zoo decks. They lack the card draw to keep up intense pressure throughout the game. That's also why you can't just nerf Doomguard and call it a day (not advocating for or against a Doomguard nerf). If they nerf Doomguard, Zoo loses a bit of power but they just swap in the next best minion (Argent Commander perhaps since they perform similar roles even if they're not as sturdy).
Personally I don't know what the answer is. Zoo is very beatable (and in this Meta, if you can't go 50% against Zoo, you need to rethink your deck) but it is annoying how common it is. Again, part of the reason that it is so common is simply because it's a cheap to make competitive deck. I do agree with the person who said that in hindsight, if the developers were to go back in time, they'd have gone a different direction with the Warlock Hero Power. I just don't think you can make a change to it at this late date without serious repercussions. Is it the best Hero Power? Without a doubt. Is it overpowered? Jury is still out.