Banlist:
FAST MANA
Black Lotus
Channel
Fastbond
Gaea's Cradle
Mana Crypt
Mana Vault
Mishra's Workshop
Mox Emerald
Mox Jet
Mox Pearl
Mox Ruby
Mox Sapphire
Sol Ring
Tolarian Academy
While mana acceleration is integral to many Commander decks, these cards accelerate mana too quickly and too early in the game, leading to uninteresting and uninteractive gameplay. Especially without the social pressure and political nature of multiplayer, these types of cards create runaway games at too high a rate.
COMBO ENABLERS
Bazaar of Baghdad
Doomsday
Entomb
Food Chain
Gifts Ungiven
Hermit Druid
Protean Hulk
Survival of the Fittest
Time Vault
Tinker
Yawgmoth's Bargain
There are a myriad of powerful combinations available in the format, but these types of cards are either essentially single-card combos (Hermit Druid milling your entire deck) or extremely efficient at enabling other combinations (Gifts Ungiven finding four cards that combine to win the game).
OPPRESSIVE COMMANDERS
Arcum Dagsson
Braids, Cabal Minion
Derevi, Empyrial Tactician
Edric, Spymaster of Trest
Erayo, Soratami Ascendant
Griselbrand
Rofellos, Llanowar Emissary
Yisan, the Wanderer Bard
Zur the Enchanter
Each of these commanders is essentially a story in its own right, but the common thread is that they are nearly impossible to beat once they resolve. There's a great deal of consistency in being able to play your commander every game and being able to cast them again should they die. In some cases, this means the deck itself becomes too consistent and too powerful, and these are commanders that have shown that to be the case.
OTHER
Ancestral Recall
Back to Basics
Balance
Humility
Karakas
Library of Alexandria
Mana Drain
Mind Twist
Moat
Natural Order
Necropotence
Oath of Druids
Serra Ascendant
The Tabernacle at Pendrell Vale
Time Walk
This is a catch-all category of cards that are either blatantly too strong (Ancestral Recall, Time Walk), or are very strong but extremely unenjoyable to play against (Karakas, Mind Twist). As mentioned above, the threshold for banning cards is lower when they're particularly frustrating to play against. All the cards in that category are very strong, but the way they warp the format is less about creating imbalance and more about making the format less enjoyable overall.
MULTIPLAYER-POINTED BANS
Biorhythm
Limited Resources
Painter's Servant
Sensei's Divining Top
Trade Secrets
While this list leans toward 1v1, it will also support multiplayer Commander on Magic Online. These are the more egregious cards in that format that aren't covered under the above categories. Sensei's Divining Top may stand out among these; this is a logistical consideration for Magic Online multiplayer.
