Emotionally, it is very similar. A direct strike, out of nowhere, on something fundamental, with a large loss of life.
Don't reduce it to "two buildings". The families don't. I don't. Al-Qaida don't when they celebrate.
Things happen we all have to deal with, and this is just as true in Star Trek as elsewhere.
Americans are NOT settling down from 9/11, but they are getting on with their lives, doing their best to deal with the aftermath.
The Federation is not tied to any one world. It, like America, is founded much more as an idea, an ideal, that has been made real.
Well I certainly don't want to margenalise what happend on 9/11. But what if that attack had been a lot worse, say not just 1000s dead, but 10s of millions? I think the anger, fear and retribution from that would have been understandably worse. In Irak probably hundres of thousands died as a concequence of the invasion which occured indirecctly as the result of what happened on 9/11. So what might have happened if 9/11 had been a lot worse, WWIII? Furthermore, the war in Iraq didn't exactly give me the impression that the American government was holding on to the ideals of the constitution all that tightly. But then again, the Federation might, maybe that's what some are interpreting as utopia in this thread.