For one, it proved to be unpredictable. The questions were culled by moderator Candy Crowley, a veteran political reporter with CNN, who looked to cover as much ground as possible during the debate. The Commission on Presidential Debates said before the debate it hoped to have about 13 questions.
Crowley asked follow-up questions, despite some initial misgivings within each candidate's camp over allowing her to do so. Her occasional interventions added another element of uncertainty to a high-stakes event.
Audience members also asked the presidential candidates about foreign policy, a subject omitted from the first debate.
The first to do so, Susan Katz, said she feared that Romney would follow President George W. Bush's foreign policy, which included two wars.
"I'm not George Bush, we are different people," Romney said, adding that "my priority is jobs." "President Bush had a very different task at a very different time."
Romney said one of his foreign policy priorities is to "get tough on China," which Obama challenged by citing Romney's time as the chief executive of Bain Capital, which invested in Chinese manufacturing companies.
"Governor Romney, you are the last person who will get tough on China," Obama said, citing his own decision to impose a tariff on Chinese-made tires to address unfair trade practices that he said created jobs.
Obama has generally scored well with the public on his management of foreign affairs, a strength he sought to drive home in May with a series of ads and appearances celebrating the one-year anniversary of the mission that killed Osama bin Laden in Pakistan.
But Romney has gained on Obama on the question of foreign policy management after the Sept. 11 attack on the U.S. consulate in Benghazi, Libya, which killed Ambassador J. Christopher Stevens and three other Americans.
The administration initially blamed the violence on protests that flared across the Middle East over a YouTube video disparaging the prophet Muhammad. But evidence has since emerged, as well as earlier security requests from U.S. diplomats in Libya, that the attacks were orchestrated by the al-Qaeda affiliate in North Africa.
On the eve of the second debate, Secretary of State Hillary Rodham Clinton said she took "full responsibility" for the American deaths, amid mostly Republican claims that Obama has sought to cover up the facts around the assault to protect his assertion that al-Qaeda is a vastly depleted force.
Romney, who was criticized for stepping in too early to condemn Obama's handling of the Benghazi attack, looked ahead of time to press Obama on the issue and his broader management of the uprisings remaking the Arab World.
But any criticism would have to be calibrated. Earlier this week, Jan Stevens, the father of the slain ambassador, told Bloomberg News in an interview that "it would really be abhorrent to make this into a campaign issue."
One questioner, Kerry Ladka, asked Obama to say who in the State Department turned down the request for extra security before the fatal attack. Obama said, as president, he was responsible for all that happened within the diplomatic corps.
He also spoke about the value of American diplomats, and criticized Romney for his early criticism of the administration over the assault.
"You don't turn national security into a political issue, certainly not right when it's happening," Obama said. "When it comes to national security, I mean what I say."
Romney pointedly applauded Obama for taking responsibility for the rejected security request, and sharply criticized the president for failing to get to discover what happened in Benghazi.
He said the violence in Libya was part of a broader mismanagement of the Middle East and weak presidential leadership that has led to tense relations with Israel, the failure to end Iran's nuclear ambitions, and wider unrest in the Arab world.
"This strategy is unraveling before our eyes," Romney said.
Obama headed into the debate looking to highlight what he has called Romney's changing positions on a variety of economic and social issues, including the question of abortion rights that is important to many female voters.
Romney has said he is against abortion, except in cases of rape, incest or when the life of the woman is at risk, but would do little to change the law that now makes abortion legal. But his written campaign platform, as well as Ryan's public statements in his debate last week, suggest that a Romney administration would work to make abortion illegal.
"I'd just note that I don't believe that bureaucrats in Washington should tell someone whether they can use contraceptives or not," Romney said. "And I don't believe employers should tell someone whether they could have contraceptive care of not. Every woman in America should have access to contraceptives."
He added that Obama was mischaracterizing his position.
The questioners primarily focused on domestic issues, including immigration reform. Lorraine Osorio asked Romney what he would do about immigrants in the country illegally who are living productive lives.
Romney said he would not grant amnesty to immigrants in the country illegally. But he said he would give green cards to immigrants who have skills that will help the United States.
When it was his turn, Obama said he had streamlined the visa process and strengthened the security presence along the Mexican-American border. But he also sought to cast Romney's position of extreme, citing statements he made during the primary season.
"His main strategy during the primary was self-deportation, making it so miserable for folks they will just leave," Obama said, adding that Romney supports the Arizona legislation that allows officials to request papers from those who look like they may be in the country illegally.
Romney responded by saying he supports portions of that law, not all of it.
"I'm not in favor of rounding up people and taking them out of this country," he said.