These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Designated Hero: The subtitle is Heroes of Warcraft. That said, two of the "heroes" are Gul'dan (an evil warlock from the franchise's backstory) and Garrosh Hellscream (the Big Bad of Mists of Pandaria).
It's possible that "heroes" is used in the sense of "controllable powerful faction leader" as it is in Warcraft (and for that matter the Defense of the Ancients: All-Stars mod)
Game Breaker: Husky Starcraft and his friend Sinvicta discovered that, using "Divine Spirit"snote "Double a minion's Health" and Lorewalker Cho, who has the odd effect of adding a copy of any just-cast spell to your opponent's hand, you can very easily get a Minion up above 1 billion health. Add an "Inner Fire"note "Change a minion's Attack to be equal to its Health and you have a very dangerous Minion. However, the utility of trick is limited, as it requires the two opponents to work together.
Explosive Overclocking: Raising the minion's health too high in this combo (specifically, past 2,147,483,647) will cause it to instantly die as its health overflows and becomes a negative number.
The combo effect on the rogue's Headcrack spell, which returns it to the player's hand if they already cast a spell. With the lack of discard and counter effects, this essentially gives them the Hunter's ability on top of their own and stacks with spell damage buffs, meaning that during the late game a rogue can be throwing a 4-5 damage spell at you every turn.
The mage's Pyroblast spell deals 10 damage to anything on the field, the opposing hero included. That means that if a player ever fights a mage and goes under 10 health, the game might as well be over, because there's nothing you can do to stop them from chucking a big, fat fireball at your face. Its mana cost was recently increased to 10 as a result.
The Twilight Drake was a 4 mana 1/1 creature that had a battlecry of gaining +1/+1 for every card in the player's hand when it was played. If a player didn't have silence to deal with it instantly, then they had, at minimum, a 6/6 running around rampant on turn 4, something nigh impossible to deal with without sinking a boatload of resources into killing it. Worse yet, they weren't legendary, meaning there could be 2 in the deck. It was later nerfed to a 4/1 that gains +1 health for every card in the hand, making it solid, but manageable.
It's Easy, so It Sucks: Veteran TCG players are quick to slam Hearthstone for being overly simple, due to the lack of playing out of turn, due to a small card pool, etc etc.
Scrappy Mechanic: The sheer amount of random effects in the game can drive players insane. For example, cards like Multi-shot aims at 2 random enemy minions, so if your opponent already has a cluttered board, pray that card will hit the minions you really, REALLY want to destroy. Or when you play Doomguard, it will randomly pick two cards in your hand and disposes of them, and you can just kiss your best cards goodbye because you don't get to choose what cards to discard.
That One Sidequest: "Class A or Class B Dominance" daily quests (example: Druid or Hunter Dominance) are probably among the most aggravating and boring quests, even if doable. They ask you to use either Class A or Class B and win five matches with them, and completing them gives you 60 Gold, averaging at 12 Gold per win. "Class A or Class B Victory" quests, another kind of daily quests, give you 40 Gold for winning just TWO matches, thus 20 Gold per win, making Dominance quests worth so much less for far more hassle. Woe to you if you keep getting Dominance quests that require entirely different classes to complete multiple days in a row, as they will SAP your time simply trying to complete those dailies. And while the game gives you an option to replace a quest you don't want, the replacement is just as random; you may end up losing a Dominance quest for ANOTHER Dominance quest.
On the other hand, the amount of gold that you can get per day is limitednote 100 from matches + whatever you can get from quests + whatever arena rewards you get.. Having dominance quests thus allows for more gold per day, which means more packs or arena runs. Dominance quests is actually favored by Arena-only players because the quest alone contributes more than a third of the entry price to said arenas.
On the other end, there's a reason why Garrosh is one of the least played characters... though he's also a regular Scrappy so more than one reason.
The priest, as well, with many players stating they'll flat out concede to a priest just to avoid playing against one.
Currently they've been nerfed to low tier but they're still not very fun to play against and tend to drag games out inordinately long (note you don't get any more gold for a long game than a short one). They're just regular scrappies now unless you're new to the game because their basic deck is among the strongest in the game.
A nerf to Mind Control has brought some of the heat off Priests. This led to mages dominating tournaments, getting the ire of the community.
Then mages' freezing mechanic was nerfed, leaving aggro-heavy Warlock builds to run rampant.