Nearly all formats in Magic have a list of banned cards. These are cards you’re not allowed to play in your deck, usually for reasons of them providing an unfair power advantage over other decks. (Cards that deal with ante or dexterity are universally banned due to creating problems with game play itself, and some cards are banned due to the decks that played them causing issues at tournaments.) Commander has its own banned list, which can be found here.
The Commander banned list is managed not by WotC but by the Rules Committee. This means the philosophy of banning cards in Commander is a bit different from the philosophy in banning cards in other formats. In Modern, Birthing Pod got banned because it was used in a large percentage of decks, and WotC wanted a greater variety in game play. In Commander, Sol Ring and Sensei’s Divining Top are used in a large percentage of decks, but they seem in no danger of getting banned.
There used to be a distinction in Commander between “banned as a commander” and “banned in the 99.” I personally liked this distinction, because it meant that if there was a card that was overpowered if you were guaranteed access to it, but less overwhelming otherwise, you could still play it. Something like Narset could be banned as a commander without being rendered completely unplayable. However, that rule is no longer in effect, so if something is banned, it just can’t be played at all.
Since Commander is ultimately a casual format, it’s up to each playgroup to determine their own banned list. If all the players in a particular group are fine with playing with banned cards, those cards may be played. The banned list is only official for officially sanctioned tournaments, such as Commander pods at a GP.
That being said, the banned list exists for a reason. The first card to be banned was Rofellos. In fact, the command tax was originally known as the Rofellos rule, and stated that if a commander cost less than 6 to cast, its controller still had to pay 6. Rofellos was still a problem, though, and eventually it was just decided to do away with the card altogether. That banning created a lot more parity in the format.
For the most part, bannings in Commander are designed to keep the format fair and fun. While the Rules Committee wants to keep to the philosophy of being able to play all your old cards that are unplayable in other formats, they also want the game to be welcoming to new players and rewarding of innovative ideas. Cards that consistently create an unfun and unfair environment for those forced to play against them are carefully considered for potential banning.
Unlike with other formats, Commander bannings don’t come out with each new set. The last one was Prophet of Kruphix, which was announced in January. That’s because the format doesn’t require regular banned list updates. Between the format not being tournament-supported, and it being multiplayer singleton, cards that are broken in other formats are much more manageable in Commander. Either their monetary value puts them out of reach of the patrons of the local gaming store, or their impact on the game is severely reduced by the fact that each player has multiple opponents who all start at forty life and are likely to take it ill if one player starts the game with a distinct advantage. Something that could win the game if it showed up opening hand in a game of Legacy will be much less advantageous in a game of Commander, as well as being less likely to show up early on. The chance of having a particular card opening hand in a typical game of constructed is about 40%. The chance of having a particular card opening hand in a typical game of Commander is about 7%. (These numbers don’t take into account any mulligan decisions that might be made.) While a one in fourteen chance of being able to play Olivia on Turn 2 due to having a Sol Ring in opening hand seems pretty good to me, it’s not going to make my opponents never want to play with me again.
Then there are cards like Shahrazad. While she’s not over-powered or anything, Commander games last long enough as it is, without throwing a subgame into the mix. Don’t get me wrong; I would love to play the card in my Judgebreaker deck, just to see the judge’s face when I ask what happens to the commanders still in the Command Zone at the beginning of the subgame.
There are of course still some cards that make the cut. Rofellos, mentioned earlier, is just way too powerful in a format that encourages large, stompy creatures that would normally be too costly to cast. Prophet of Kruphix allowed its controller to basically take a turn on each other player’s turn, the only mitigating factor being the lack of card advantage. Then again, an Azami or an Arcanis on the field would create or even increase said card advantage. The end result was that if the game wasn’t over after you dropped Prophet of Kruphix, it was because you’d built your deck wrong.
That’s not the sort of play style we want to encourage in Commander. It shouldn’t be about whether you can draw or tutor into a specific card. Commander is about fun interactions that come about when you have multiple decks trying to do disparate things all in the same game. We don’t want one player taking up half the time we’ve allotted for the game. We want everyone interacting and having a good time.
So there you have it. Some cards that are super-powerful in other formats are much less game-changing in Commander. And some cards that are fine in two-player suddenly become oppressive in Commander. The banned list reflects this. Don’t be surprised to see cards that are banned in Legacy or Modern are legal in Commander, or vice versa. And don’t make the mistake of thinking that just because something is a bomb in another format it will be equally good in this one.