I guarantee you, most Americans had not even heard of Osama bin Laden or al-Qaeda prior to 9/11. Even Islamic terrorists were seen through the lens of 1970's and 80's paradigms, where they were viewed as nationalists like the PLO or Hamas or Hezbollah who happened to also be Islamic, not Islamic terrorists.
I think a lot of that began to change with the beheadings. The beheading of Nicholas Berg shocked the fuck out of a lot of people. It seemed so far beyond the pale. I mean, shoot a guy dead, yeah. But saw his fucking head off with a big knife while he's screaming and you're videotaping it for YouTube? Even Hollywood evil didn't go that far.
The Taliban we could still sorta look at as Afghani nationalists who happened to also be fundamentalists. But al-Qaeda was stateless. They were everywhere and nowhere. They weren't fighting for a particular chunk of land, they were waging global jihad. That was a new thing, and it scared the hell out of a lot of people because it suddenly meant potentially nowhere was safe. Americans weren't scared of the PLO or Hamas or Hezbollah because as long as you weren't planning on being in the Middle East, you probably weren't getting caught in the crossfire. It's the same logic that allowed a lot of Americans to sympathize with the IRA.
Hell, in some ways on the surface Daesh is a return to a simpler time when we could say "hey maybe these guys just want a chunk of land and will leave us alone if we give it to them, or at least stay out of their way". Even just a few weeks ago, Trump had stated during a debate that he didn't understand why we were hung up on ISIS and we should just let Putin deal with them. Then Paris, and now he's stopping just short of making American Muslims wear a crescent armband and report to the camps.