Warrior Deathknight isn't bad, it's quite a solid card. It's just not a card that screams "omg broken card pls nerf". Basically, it's an 8 mana, gain 5, deal 4 damage to up to 3 minions, take some damage, change hero power. On demand whirlwind for 1 mana more isn't a bad hero power by any stretch of the imagination, as warrior has enough cards to support that ability.
It's problem is that quest Warrior is still a thing, and Deathknight Warrior wades a little into the same territory of midrange/control. So it's not that it's bad, it's just overshadowed by other decks already in existence.