"None of it means it's over, Afghanistan is a democracy, and we've won," said Ronald Neumann, a former U.S. ambassador to Afghanistan. "But I don't think you can look at this turnout—in the rain and against death threats—and say nothing much has been achieved, as critics like to say."

One of the biggest beneficiaries was the Afghan security forces. To the surprise of both Afghan and foreign observers, the Taliban failed to carry out a single large-scale assault in a major city. The group claimed to have carried out 1,000 attacks nationwide but security officials said that was a gross exaggeration. Election observers said the level of violence was unclear.

This year, there were about 90,000 fewer American and NATO troops in the country and those that remained were confined to bases and served as a reserve force. Instead, 350,000 Afghan police and soldiers fanned out across the country. Wilder said the Afghan government security effort in Kabul, where he observed the vote, was the most sweeping he has seen in 30 years of intermittently working in the country.

"It was a really phenomenal security operation," said Wilder. "I've never seen anything like it." After the vote, Afghan police were a "sensation" on social media, garnering wide praise, according to Wilder. Their popularity also extended to the street: One election observer reported seeing a group of young Afghans buy scores of roses and distribute them to police officers the day after the vote.

Barnett Rubin, an expert on Afghanistan who served as a senior advisor to the State Department from 2009 to 2013, cautioned that the Taliban will retaliate. "There will be a test of strength this year and next year," Rubin said, referring to Taliban attacks. But "an election that goes well can only strengthen the morale of the security forces and reduce the morale of the Taliban."

The news media—as well as social media—played an unprecedented role as well. A series of presidential debates broadcast live on television were more sophisticated than in the past. And intensive Afghan news coverage—and social-media commentary—on Taliban attacks before the vote may have driven up turnout.

In particular, the pre-election, execution-style killing of popular Afghan journalist Sardar Achmad, his wife, six-year-old daughter and four-year-old son as they ate a holiday dinner in a Kabul luxury hotel outraged many Afghans. A Facebook page that tracked the recovery of a two-year-old boy who was shot in the head—the family's lone survivor—generated vast online traffic and sympathy. "The attacks really mobilized Afghans to come out and vote," said Wilder.

And in terms of candidates, it was Ghani, the technocrat-turned-effective campaigner, that most surprised observers. Despite wide praise for Ghani's 2002-2004 tenure as Afghanistan's finance minister, he was seen as lacking political skills or a large electoral base. Afghans who remained in the country during the Soviet occupation in the 1980s and the civil wars in the 1990s seemed to resent Afghans who, like Ghani, had fled the country and flourished. In 2009, Ghani ran for president and won about 4 percent of the vote.