I feel as though this forum needs a thread like this that really identifies the main difference between what constitutes as "Overly powerful" and just "Powerful."
In my opinion, cards like dim shield in vanilla are absolutely not over powered. It's a powerful card, but calling it over powered is totally not fair. Tons of things get rid of dim shield; momentum, explosion, Midas, steal, pulvy, BE, poison, silence, spell damage, and probably more that I can't think of. On top of that, dim shield sucks vs a lot of stall decks. If your deck doesn't pack at least one of those things and you get mind gated, that's your fault, not the cards fault.
So, why nerf it? Well, I feel like maybe there is a lingering mentality that just because a card is powerful means it's OP.
Another example of a good card that isn't OP is Blue nymph. Blue nymphs are arguably one of the most powerful creatures in the game, being able to essentially deal 26 damage a turn at the cost of 8 quanta plus 4 extra a turn. Now, this might seem incredibly good on paper, but these nymphs are incredibly slow and while not fragile, are definitely vulnerable to CC. They die to lightning, take a major hit from RT, cost a lot, can be lobo'd, UG can be reflected, and most importantly can be easily out rushed. This card does not deserve to be nerfed, it is a very well designed card and just because it is at the top of the food chain doesn't mean it's too powerful.
(Additionally rather then nerfing cards, buffing other cards in the past has proven to be far more effective since it can potentially kill two birds with one stone. Sosac used to be incredibly broken, so Zanz buffed very weak card and it suddenly became a staple in arena. That card was purify.)
However, as you know, there was a point where several shards really DID need a nerf. Sosac used to be usable in almost any popular deck, same with SoG, SoB, and SoFo. Have these cards become any less powerful after the nerf? Barely, SoG took the biggest hit of all (and I even consider that to be unfair from my perspective). They continued to be powerful and used in many decks and still are today, so then what was the point? Well, they were broken, they functioned in a way that the game is not compatible with and faced the same problem that countless other overpowered cards in other games faced: they were too easy to use. Cards that are super easy to use but still incredibly powerful are the cards that truly deserve nerfs, not cards like dim shield and blue nymph who require you to build the deck around them due to their heavy cost.
Another example is sundial. Remember when sundial lasted two turns? The damn thing was free and yet it was better than dim shield which cost 5 upgraded. If sundial cost 4-5 time quanta, it would absolutely not be broken, but the thing was that it was free and able to be used in almost any set of cards. In its current form in vanilla, it's just a very good card since it was nerfed.
Sundial and shards is also a good example of HOW to nerf something. Increasing the cost or decreasing the effectiveness is a bad way to nerf something(unless it was implemented for testing purposes). Making them harder to cast cast/use in interesting ways is far more productive. Zanz added a nerf to SoG requiring life mark and making it require life quanta, which is far more effective(and interesting) then just increasing the cost or decreasing the effect. Sundials were handled a little differently, but what effectively happened is they became a one turn stall button rather than an almost-better-than-dim-shield rip off, which might seem simple but reducing the effect by one turn more less changed the way it was built into decks in the same way the other cards are.
Summary: Just because a card is super good doesn't mean it's broken. Nerfing cards is bad and promotes degeneracy, implement well and nerf as little as possible. Before nerfing something, consider buffing something else first in order to balance it out. If something is inherently broken due to its easy accessibility and powerful nature, nerf it in a unique or interesting way rather than changing the cost or effectiveness.
I would like to say that some of the nerfs/buffs I've seen aren't all bad, I liked the approach to nerfing sosac and SoD in order to make them more tournament friendly. But some of the other approaches don't seem quite as good, I'm not big on the dim shield nerf nor am I fond of the nerf to SoG(although, this wasn't "exactly" a nerf, I still don't like it).