The main things they said upfront that they needed to change was that it needed to be set in America rather than Europe, because Americans wouldn't watch a show set in Europe. Particularly it needed to be set in Los Angeles, New York, Chicago, or... one other city that i'm forgetting (Boston maybe?) because those are the only cities Americans are interested in seeing on TV. Also, in the book the flashforward was twenty years, but it couldn't be that long on TV because that would mean everyone would be old during the flashforward segments, and Americans are only interested in seeing young and beautiful people on TV. (They originally suggested the time should be shortened to one year. He immediately responded that it should be six months instead. That way the different season would help clue people in on when it was a flashforward segment, an idea that was foreign to the producers who were based in Los Angeles =) Finally, they said they were going to have to change the professions of the main characters, because Americans weren't interested in seeing a story about three physicists and an engineer. The only professions Americans want to see on TV are cops, doctors and lawyers.
He agreed to all of those conditions, however he was very amused when he could occasionally take the afternoon off from the set of FlashForward to go visit the set of Big Bang Theory, the show about three physicists and an engineer that was kicking their ass in the ratings :)
Overall though he seemed fairly happy with the way the series turned out (despite a slight overemphasis on kidnapping as a plot device =) so i might go check it out again. shelleycat and i never made it past the first two or three episodes when it was on the air.
Also, when i asked him about what he thought about the way the world has actually turned out in 2012 compared to how he first portrayed it in his "Wake" "Watch" "Wonder" trilogy (which he started in 2006) he talked a lot about the rate of technological change, and the value of "pop culture" references in books. But he also mentioned that if he had the opportunity to go back and change anything about the books now, the one thing he'd change is that he wouldn't make the protagonist's blog be on LiveJournal =P