The healthcare bill Massachusetts Gov. Deval Patrick unveiled yesterday leaves many key questions unanswered, said business leaders, industry executives, and legislators, and how those issues are worked out will determine whether the proposal reins in rising costs as it promises. The milestone legislation, which Patrick announced before a standing-room-only breakfast meeting of executives at a downtown Boston hotel, is the most comprehensive attempt by a state to transform how providers are paid and curb costs while improving care for patients. The governor said he wanted it to be a national model. Patrick was emphatic and energetic in a speech widely viewed as crucial in setting his second term agenda.