Luck of the draw
There are always luck elements to CCGs, down purely to the card-draw mechanic. The biggest here are Prophecies. Every five health points you lose unlocks a free card. If it’s one with the Prophecy effect, you can use it for free, in the middle of the other player’s turn. (Drawn naturally, it has a cost like any other.) It might be a free minion, or something like a fireball spell that will take out the unit the other player just put down. Players missing elements such as Magic: The Gathering’s ‘declare blocker’ stage may appreciate the additional nudge away from Hearthstone’s pure turns, but mostly it’s an annoying mid-turn distraction that can’t really be played around or predicted well enough to feel like a tactical addition.