I just wanted to ask why the card is banned. I always assumed it was too easy to go infinite with if you had an extra turn and that such a combo could be played early at 5 mana over two turns or ends the game at 10(discounting Savor the Moment) while giving the table a turn cycle to get rid of the thing or the player.
And then it dawned on me that the banlist is made for fun, casual EDH and the Mirror combo needs a bunch of extra turns in the deck to end the game very early.
When you're playing a whole bunch of extra turns you're not playing the fun, inclusive casual format anymore. A casual player can't crash the game early with Panoptic Mirror combos. Besides, there is a rather long list of comparably strong combos that don't need a specific deck and are found in a lot of metas like Basalt Monolith+Rings of Brighthearth(or Helm of Obedience+Leyline of the Void/Rest in Peace/Planar Void or Buried Alive for Mike+Trike plus any reanimation spell, I could list more if asked).
I don't assume winning the game for 10+ mana and a two-card-combo that can be disenchanted before anything happens is a real problem, Expropriate will do the job with a single card in the same color and there are enough other legal options in other colors that require little build around like Tooth and Nail or Boonweaver Giant that end the game now, not next turn.
And lastly with all the cards designed for EDH play we've gotten over the past couple years the non-combo strength of the card pales more and more. Today everyone has easy access to stuff like Paradox Engine, I'm not gonna throw around any more names nowdeadeyenavigator.

Is it a strong card? Absolutely. Does it gain you a lot of value if unchecked for multiple turns? That's why I wanna play it! But so does Survival of the Fittest which I find just so much stronger(and that also ends the game rather quick and early by the way).
So what would we have to be afraid of that isn't already part of casual tables? A value engine? Cultivate every turn starting on T6 or 7? A wonky budget Abyss? Build your own Debtor's Knell? Let me know what it is!

Howdy! This has been discussed at length a number of times already, which you may not have known. There's a lot of people on both sides of the fence, so if you start searching for threads on here you're very likely to run into a bunch of these discussions.

It really comes down to just a few things: it's colorless; it turns virtually every instant and sorcery into a powerhouse when left unchecked; most effects repeated for free become quickly backbreaking; and its an artifact.

If Panoptic Mirror weren't colorless, there's a possibility it wouldn't be banned. Being colorless allows it access to every instant and sorcery in the game, and being able to couple with every powerful effect in the game is somewhat problematic (and with more releases, the pool of powerful effects continues to grow). That being said, it makes every effect powerful--not just the powerful ones. Getting a free every turn sounds fun, but it quickly becomes backbreaking when you suddenly never have any mana problems and quite a bit more mana than anyone else. Simple draw spells suddenly push you way far ahead in card advantage. The most basic kill spells keep opponents from ever having creatures again. It's arguably a very fun situation to be in...but only for the person with Panoptic Mirror.

And finally, being an artifact makes it easily steal-able and recurrable. It centralizes the game to being about who has it longer, or get it out more quickly, or protects it better (or all of these). Which swings us back around to it being colorless--most decks will need to consider running it, because literally every deck can run it (even those without a whole lot of spells). So in conclusion, it's very likely to fall under "Creates Undesirable Game States" and "Problematic Casual Omnipresence."

Not everyone agrees with these points or may have different opinions about why it should or shouldn't remain banned, which is fine. It is a card that gets discussed often.

Thanks! I ran a quick search but wasn't really satisfied with what I'd found.
That summary you provided looked sound when I first read it but when I compare it to effects that are being played already it doesn't seem that strong anymore(I'm writing this for the RC more than anyone else really).
Boundless Realms is arguably stronger than a ramp spell in the mirror and that's not a banworthy card. Playing against Grimgrin or Thraximundar can be annoying but they don't keep three opponents from having creatures, mostly just one and some decks will just hold against it. Draw spells are a different animal but I'll just claim here that people rarely play instants and sorceries that say "draw [n] cards" outside of blue draw-go decks which are a tiny, tiny fraction of the format, also Consecrated Sphinx seems to be an ok card(while it is a creature it draws way more than the mirror would ever do and is easy to recur). Only blue really has these specific spells and card advantage comes in many forms like ETBs or engines built with permanents.
I also don't think that artifacts are easy to steal. There is Steal Artifact, five blue enchantments that steal permanents and cost 6-7 mana, Dack Fayden, Memnarch and three old, obscure creatures that never see play and can be Shock-ed to end the effect and that's it. I've had that argument about Emrakul once and I got it, but this is not Emrakul and it's not Prophet of Kruphix either, it doesn't explode in your face when it comes back and requires mana and a specific card type in hand to start doing anything.
Now is it easily recurrable? A search for "return target artifact card from your graveyard" yields only 20 results, bare in mind they are not all playable and few return it to the battlefield. Once you've achieved that you got to have another spell in your hand and pay mana again, that's a lot of time you'll be spending on that one time recursion, not talking about repeatable recursion which is pretty hard to achieve for artifacts and not within the realm of casual or omnipresence.
Now the argument about Casual Omnipresence. I'd say an average deck runs around 15 instants and sorceries, just from looking at tappedout a lot in my time and casual decks will tend to run even less. In a game that goes to T15, if you manage to draw 10 extra cards, which I find generous(even more so as an average for casual tables), you will have seen 32 cards. That's 5 instants/sorceries you get to see in a long game with a lot of extra cards, total, at the end. You can bend the numbers a bit if you like but it's not changing a lot. That doesn't sound like people are going to be cramming Panoptic Mirrors into all of their decklists.
For those reasons I think the case against Panoptik Mirror is a lot weaker than it looks and it deserves a chance for scrutiny.

On the case of "not as easy to steal"... kinda. There's also a few sorceries, and if you can make a copy of it, things get ugly.
As for really strong cards that can be imprinted under a Mirror-- ramp means you never need to really make another land drop. A board wipe or any other removal is awful. An extra turn card ends the game.
Sure, I would use the Mirror if it were unbanned. Then again, I can say the same for PoK or Prime Time-- but that does not iply a balanced status

For me it boils down to this:
1. It's incredibly easy to break, so much so that people will do it by accident. Even very benign applications provide very large advantage.
2. It's incredibly powerful meaning it will be played often, in as many decks as possible and will be a focus of any game it is in (copied, stolen, reanimated)
3. It's colourless which means that it will go in any deck which wants to run it, which compounds the above problems by putting it into a large percentage of decks.

Ubiquity and power, almost a perfect recipe for a card being banned.