• Amalia Benavides Aguirre and Sorin, Imperious Bloodlord are banned in the Pioneer format.
• Nadu, Winged Wisdom and Grief are banned in the Modern format.
• Grief is banned in the Legacy format.
• Urza's Saga and Vexing Bauble are restricted in the Vintage format.

Starting with Pioneer, WotC essentially just nerfed the top two decks in the Pioneed format, Rakdos Vampires and Abzan Amalia Combo.  Rakdos decks featuring Sorin had basically made up about 36% of the Pioneer format. Amalia decks were further behind at 12% of the format.

In Modern, the Bird was an obvious ban after dominating the Pro Tour (see "Cumbersome Combo").  Grief was also a longtime problem card that finally got banned, as it makes players who mulligan suffer further by removing one or more cards (if flickered) from their hand for free.  Grief was also banned in Legacy for the similar reasoning.

Surprisingly, WotC didn't ban The One Ring, even after Mark Rosewater mentioned it as an overpowered card (see "The One Ring Card Was Polarizing for Fans ").  Designers said the following about The One Ring in the announcement:  "While present in several decks, there is no clear The One Ring deck terrorizing Modern.  Being a unique combination of self-protection and card advantage, it is a strong card that helps prop up several varied strategies."

Finally, Urza's Saga and Vexing Bauble are restricted in Vintage.  Urza's Saga was restricted as a metgame decision, and Vexing Bauble was restricted because it shuts down the core cards in the format on turn one (Black Lotus, Moxes, and more).