Re: what changed?
A fair amount actually.
1. Stepped up intelligence actions. For all its invasiveness the NSA wiretaps. I may hate them and call them unconstitutional, but they certainly have been used to disrupt some plots.
2. They killed 3,000. Then like the wrath of God we came after them and killed even more. You hate us for it and criticize it, but it works.
3. The 9/11 attacks themselves changed the existing paradigm. Before it was: surrender to the hostages and most of you will get out alive. Now it is: kill the hijackers by any means necessary before they kill any of your fellow citizens; you might die doing so, but you will die if you don't.
4. This is the US. You hate us and criticize us for our gun love. But outside of gun-free government zones (like schools, airports, and airplanes) you don't know who might be packing heat. One well placed and moderately well trained cowboy could ruin your whole plan. You might not even get your 72 virgins. Because he just might be the sort of mean, racist f*ck who'd maim and disable you before shoving a pork sausage down your throat to choke you to death.
I'm sure there are more things, but that should get you started.