There are many ways in which the United States changed. Let me mention three of them.
First, aviation rules have changed. Anyone who has travelled has seen that security has been ramped up. Also airplanes now have fortified cockpit doors and pilots can now apply to become federal flight deck officers, which means they can carry guns.
Second, the Patriot Act was introduced and at least 48 bills and resolutions have been approved and signed into law. Some of them include: Enhanced border control, tighter issuance of Visas, and even things like health bill for those who worked in ground zero after 9/11.
Third, there have been more hate crimes and suspicion when it comes to people from the Middle East. Hopefully this will change, but the racism seems to increase.
In conclusion, we live in a nation will more suspicion, security, and fear.