Meet Valar Morghulis:
First, let's introduce Valar: a 1.0 plot that read "When revealed, kill all characters in play", with 2 gold, 0 initiative and 0 claim.
It was a staple of pretty much every control deck, most toolbox decks, a certain brand of aggro (Greyjoy), and many combo decks. It was printed in the 1.0 core, and was a defining element of the game from then on - you could safely assume your opponent was running a copy, and play a meta-game of delaying until your opponent was forced to Valar themselves, or luring out a Valar early in order to exploit the post-valar space. It was an interesting card, and it defined the first edition for a long time. That doesn't mean all good decks ran it (or should), but it's probably fair to say that it was in 80+% of decks (and deserved it's place in about 75%).
It made intrigue challenges generally better than military challenges by punishing the person who relied too heavily on characters on the board and couldn't refill the board, card draw even more critical than it already was, saves better in general, decks built around triplicate copies of characters worse... the list could go on, as so many elements were shaped by this.
The sentence that has often been repeated, and triggered (initiated?) this rant is that Valar was a "comeback card". Huh? I think you're mis-remembering.
Valar doesn't inherently make the game swing back and forth more, it doesn't make it so the person who is "losing" gets to turn the game around, etc. In GJ or Brotherhood, you were playing Valar aggresively - pushing an advantage (in saves/immunity) you had. In a control deck, you were playing Valar defensively, leveraging your location base/hand size/card draw engine as superior to your opponent's. In combo decks, you might be playing valar to eliminate disruption of your combo.
These are "leverage". In all of these cases, you're saying "My non-character tools are better than your non-character tools, so let's wipe the board and pit those against each other". You are leveraging hand size, card quality, draw engine, location base - and doing so against character board presence. That's not a comeback - within the state of the 1.0 game, it's pretty obvious to a veteran player who is in the lead, and that's a decision that factors in Valar. It just means that "who is in the lead" is not solely defined by bodies on the board. Saying Valar is a comeback card, to me, is akin to saying "I got to 13 power before that Bloodthirst game turned the game around! I almost had it!". Naive.
Between equally skilled players, Valar doesn't help you come from behind if you're out-drawn 2 to 1. It doesn't help you come from behind if your opponent has thrice the economy you have. It doesn't help you come from behind... at all. In the vast majority of cases, if you think your "Valar turned the game around", you were probably wrong. And you're probably not remembering all the times you Valard but failed to affect the outcome. Sure, maybe Valar delayed the inevitable - but in the slower core 2.0 environment, do you really want to take 2-3 more turns to lose?
Don't get me wrong:
Valar was great for first edition. It created tense turns, skillful play, a bluffing game, a powerful sense of impermanence, and made you look at considerably more than board state when evaluating a game. It might even be just as good for second edition (reserve values, bouncing attachments and a limited economy make it very different, if it were to re-appear). I'm just asking that you don't make it into something it wasn't.
We have "leverage" plots; Game of Thrones leverages intrigue icons, Fortified Position leverages raw STR, Jousting Contest, Wildfire and Calling the Banners leverage against swarms. Supporting the Faith leverages against challenge surprises, Naval Superiority leverages your economy, Power Behind the Throne leverages your best character, Filthy Accusation leverages against theirs. Those cards change how you evaluate the GAME state (in this case, often the board state) and find out who is winning. They do so by making certain elements more important than others - which is what Valar did.