Why did the Greenland-Forest City deal take a year? "Complicated" likely relates to 421-a revision, at least
That veto power related to the board structure of Greenland Forest City Partners. In March 2014, Forest City told the SEC:
... the Brooklyn Atlantic Yards project will be managed by a board composed of three representatives from Greenland and two representatives from the Company. While decisions would require a majority vote, many decisions labeled “Special Major Decisions” would require a vote by us for approval. There is the risk that many of the decisions made by the joint venture would not be in our best interests and, further, that an inability to agree on certain of the Special Major Decisions would trigger buy-sell rights and obligations between us and Greenland. The exercise of the buy-sell rights could result in our having to fund the purchase of Greenland’s interest in the entire joint venture, or in one or more individual parcels. It could also result in having our interests be purchased and the loss of ownership of the Brooklyn Atlantic Yards project or of one or more parcels thereof.That suggests that, if Greenland disagreed with Forest City's stall, then it could have bought out Forest City's share of a specific building site or a larger share. The latter seems to have happened.
(Given that the upper limit is 130% of Area Median Income, or AMI, and a significant chunk of Atlantic Yards/Pacific Park units are above that, it's certainly possible the new AY/PP buildings will have more "affordable housing" than the minimum. That's because the developers face a 2025 deadline for 2,250 units of affordable housing--defined as participating in government subsidy programs--and have built only about 782.)