The player that has the option of playing this card has an advantage though because they can build their deck around it and choose both when to play it and how. The opponent most likely isn't going to have a deck that takes advantage of an upgraded HP, and depending on their class may not even gain much of an advantage because of it. Giving a slight mana penalty to the player who is using this card is balanced.

That being said it's a Druid card and they have one of the worst hero powers when it comes to upgrading or discounting so if there's one reason this card would be bad it would be because of that. Would pretty much have to be used to fit into a hero attack druid deck (which also doesn't use DK Malfurion) which with current cards is still bad. So unless Blizzard continues to push that archetype it would still never be played.