Also playing Quest Rogue again I really don't get the nerf. I'm barely winning half my games.
I think Kibler explained the issue well in his
latest video for Omnislash, but I'll try to break it down here.
1) The deck is extremely popular. That isn't an issue in and of itself, but because it's so popular, people are now teching against it -- as much as you can with such a non-interactive deck. Essentially, that means more aggro. Despite how much people try to counter play it, it still sees a pretty good win percentage. That's an issue. If people are trying to counter a certain deck and still fail often when playing against it, there's a problem.
2) It's repetitive. Because it's a Quest, it always comes into play. The monotonous nature of playing against QR, and the fact that it makes all of their minions the same, stat-wise, adds to this. It makes for boring games that feel like they come down to whether the QR got a good draw or not. Sometimes you can play around it and use skill/tech cards to squeeze out a win, but more often than not, it's all about who draws what -- moreso than usual.
3) Because of 2, it's just not fun to play against. You don't feel in control at all, and the effect is both boring and obnoxious. It's the kind of deck Blizzard should fear, because it can cause people to quit the game entirely. In fact, IIRC, Noxious quit the game (which was making him considerable $$$) to play Gwent instead, partially thanks to QR.
I actually don't think the change will completely kill the deck, but I think it will diminish the amount we see it -- just like when Pirate Warrior fell off after the STB nerf.