Zendikar Rising has only been available for one week in Magic Online and Magic: The Gathering Arena, but one deck has already proven to be powerful enough that Wizards of the Coast has decided to intervene. The Four-Color Omnath deck, which revolves around Uro, Titan of Nature's Wrath, Lotus Cobra, and Omnath, Locus of Creation, has proven to be dominant in Standard play. To that end, Wizards has decided to ban Uro, noting that even the decks that attempt to counter the Four-Color Omnath deck are utilizing this particular card.
Wizards noted that this is a very early ban, but Uro has shown signs of being troublesome in the past as well. Furthermore, the move to mostly-online play means that the metagame evolves more quickly than before. Wizards decided they want players to be able to experience Zendikar Rising as much as possible in this specific period, and banning Uro will hopefully help to achieve this goal.
This ban will be effective immediately. The full Banned & Restricted list can be found here.