An aggro deck should not be allowed to play the exact same way against all of its competition. That's just how card games work, it's all just a very complicated game of rock-paper-scissors.
Nor am I saying it should, nor am I calling for a nerf that reduces the positive effects of Flamestrike, nor do I claim that my deck or any other deck should work against everything.
Why is this the case? Like, why did their minions survive the prior turn?
Come on now, there are a lot of situations in which destroying all minions your opponent plays within one turn is not possible or viable.
Flamestrike does a good job of clearing the board of trash minions. That's the only thing it does, however. It's a bad card if the board can persist (Divine Shield, Deathrattles) or just survive it outright (Yetis). For the longest time, Flamestrike was a dead card in Constructed play. It literally did nothing at mid-to-high levels of play.
If you consider all 4 health and below minions and all minions that have been damaged to 4 health or below 'trash', then I just have nothing to say to you except how wrong you are.
Divine Shields are only really viable when playing Paladin, and in most cases Divine Shields have already been removed from your minions in prior turns to playing Flamestrike. Deathrattles are useful (I have Harvest Golem and Leper Gnome in my deck), but will not turn around the game in your favor when you've just lost all your minions and only gained a Damaged Golem for it. And again, to have enough Divine Shields / Deathrattles to 'counter' Flamestrike, you'd have to place several specific cards in your deck for one single specific card of one single specific class that they may or may not play.
And to all of you; stop claiming that the 'opponent would have won anyway'; not being able to kill two enemy minions the instant they are played or not dealing with them in that turn because you choose to deal direct damage (taking the risk that they buff their minions next turn) is not the same as the opponent 'having board control', especially when I have two 4/4 taunters on my side of the field and a 2/3 effect minion. Even if they had no minions at all and you have full board control, Flamestrike will destroy that board control instantly with little to no drawback for the opponent. One single card should not do that when played on it's own.
And again;
not asking for a removal of the card or a reduction of it's positive effects; just for a slight increase in cost or a small bit of text that details a small negative effect. If I played my current deck against it, it would still destroy me, just like the card is doing now, but again, that's not the point and that's not why I think the card should be nerfed in some way.