Game balance and Blizzard, that's a long love story right there! There hasn't been a lack of changes on Hearthstone between the beta phase and now. The targets have largely been the cards that could be found in absolutely all the decks. The problem has rarely been the choice of cards to modify, which was often justified, instead the strength of some of the nerfs. This is how certain cards have gone from being practically guaranteed in a deck to having almost zero influence!
Even if it is still a must have for some, its nerf has certainly reduced the potential of this card. Today it is no longer a 4/3 but a 4/2. The impact of this change isn't huge since the divine shield is there to take the shock. However the bubble is no longer on the creature, its 2 health points don't give it any chance of survival, especially in turn 6!
A slight nerf for this card which is still interesting but which has had its impact seriously diminished. It was a justified change made to the battlecry which no longer provides a +2 attack to a minion permanently. The effect is now temporary but it still allows the player to get rid of a big creature or attack the health points of the enemy hero. Obviously the card was stronger before, with, as well as the stats of the Dark Iron Dwarf, a permanent buff that made a creature a lot more powerful.
This legendary was in most of the competitive decks because it was very difficult to counter! If it was played in the early game, it often gave you a huge Card Advantage. It no longer allows you to draw at the end of your turn, only at the beginning. So its effect is no longer immediate. The "draw card' is obviously still random and subject to the will of Nat. The card is now not as strong but this nerf allows it to be viable and remain a permanent choice.
Once again, this was a legendary present in most of the decks. For a long time it was the first legendary to craft, so big was its role in all the games. But yes, its mana cost has gone from 5 to 6. An important change because the card slows down the game a lot and other alternatives like Cairne Bloodhoof are now preferable. The card is still useful but it has clearly lost its shine. It now only finds a place in the late game decks, and even then...
It was present in almost all the decks of the game, whether it was a basic deck or those played in competition. It was a 1/2 that allowed the player to draw. Its battlecry hasn't changed but it is now a 1/1. That sounds like nothing, and yet it is a big nerf! The Novice Engineer has now disappeared from most decks. Preferable instead is Loot Hoarder and/or Nat Pagle, as well as the 'draw cards' specific to each class.
This was long ago when the Priest still dominated the ranked games and the Mind Control only cost 8 mana! So it was still possible to do something else in the turn, such as use your heroic power or play a little creature. That time is now over since the card now consumes all the mana you have in turn 10. So the card has lost a lot of its usefulness, the solution being to only have one in your deck, to avoid being stuck with two cards that have become very/too late game.
The Mage had already seen the mana cost of some of its spells increase, but nothing compared to the nerf of "Pyro". Once, the aim of the Mage was to bring down your health points to 10 and win in turn 8. Some matches were over too quickly due to a card that was able to remove 33% of your HP without you being able to counter it! As with the Priest's Mind Control, the Pyroblast, too much of a gamebraker, has had its mana cost increased by 2. As such it has become difficult to integrate it into most of the Mage decks.
This was an essential card for those playing as the Warrior. Decks like the OTK Warrior (One Turn Kill) were even built around the Warsong Commander! It used to grant a charge to all the other minions. When played right, it was possible to deal a huge burst to your opponent. The card now only gives the charge to creatures with 3 attack points or less... Once again it was a very brutal nerf that has killed off the use of this card.
This is one of the many 'destructive' nerfs. Not only have its stats gone from 1/1 to 0/1, but is ability has also taken a massive hit! While the card is still a stealth, it no longer gives +1 HP to all minions but only to one and it is randomly selected. No need to spell it out, the Blood Imp, used in the Warlock aggro decks, is dead!
This simply was an indespensable legendary that integrated most of the competitive decks, notably those based on control of the board. The card had weaker stats since it was a 2/2 but it was its ability that made it strong! You could choose to transform an enemy minion into a 5/5 devilsaurus or a 1/1 squirrel. All you had to do was make sure the results went in your favour. So if this card was used on Sylvanas or Ragnaros, the transformation could only be good. The card has now lost almost all its use since the effect is now random! Put more simply, Tinkmaster Overspark is no longer a useful legendary. Preferable instead is Big Game Hunter for the big cards or The Black Knight for imposing taunts.
Hearthstone Guide: Presentation of the Arena Mode
HearthStone: Top 10 Free Basic Cards
HearthStone Guide: Miracle Rogue Deck!
Guide Hearthstone: Daily Quests and Achievements
HearthStone Guide: Advanced Warlock Murloc Deck!