One panelist's first idea is what if Eisenhower didn't get his Interstate System through Congress. It was a really messy battle. What would the effect being on our psychology if it was normal to have to putt along at 30 mph through the country? Another panelist pointed out that Route 66 was already acquiring a positive mythology by that point, and Route 1. Higher speeds would have come, just gradually. Perhaps not fully even to this day.
What if Princess Charlotte had not died in childbirth, and the child had lived, too? From the audience, I pointed out that this one is hard because it turns on the question of what sort of person the survivor would have turned out to be. What if Prince Arthur had lived to be king? Or long enough to get Catherine pregnant? What would he or his son have been like? (Long enough that Henry VIII had married someone else when Catherine was widowed would not raise that question.) Others pointed out that -- would it have made a difference? One cited Lord Darcy, which I think is weak. Why would Richard the Lion-Hearted's settling down and becoming a good king make magic be codified rather than science? It's a hand-wave, nothing else.
The problems of medical treatments and getting them to work better.
What if the Assyrians had gone on to thoroughly trample the Jews into non-existence when they were first there? Harry Turtledove's stories in which Mohamed became a Christian, a monk, and a great inspiration. (A panelist took the occasion to point out that that sort of choice could raise offense. Surprisingly enough, one audience member wanted to know why.) One panelist argued that Constantine made a difference, you could make Christianity a little sect that way. I pointed out that, on the contrary, Constantine's successor was an Arian, and his successor a full-blown pagan. They were not calling the shots. But in the very program description, there was one: Poul Anderson did one in the Punic Wars. Scipio Africanus and his father died early, Carthage won, Rome wasn't around to provide countering influence in the times of the Maccabees, Judaism ended, Christianity never started, and so neither did science. I pointed out that Anderson's characters were specifically trained to unravel things like that.
One panelist took off on that, on the whole notion of how many assumptions are necessarily built into any alternate history and its string of causes and effects.
One audience member wondered what we would guess had been meddled with in our own history, if we had evidence of a Time Patrol of some kind. Very odd, unusual things, they concluded. Like the famous lost order of Lee's that fell into McClellan's hands. Even McClellan, who could usually pull defeat from the very jaws of victory, only managed a bloody and bungled victory at Antietam, rather than a full defeat.
Vinland might have succeeded if the Vikings had managed to change their diets and not tried to raise cows in lands unsuited for them. One panelist discussed another author, who had the notion of feudal castles in Connecticut and tried to work back from there. Viking success was one way. The Aryan migration going the other way was another, a la Lord Kalvan of Otherwhen. (More discussion of that book, of course.)
Much discussion of the Mongols or the Japanese in America. If the Mongol invasions of Japan had worked -- and only weather had foiled them -- imagine Japanese going up by the currents on the shorelines, and ending up in America. Anderson did one about the Monguls themselves finding it -- and his characters discovered that they were not preserving a pure untainted past, but the one that would lead to the Time Patrol. The Monguls would have succeeded without their meddling.
Concluding observations that the genre was no longer really SF or fantasy -- it was a genre of its own, having cast off its original connections.