Well, a lot of people died, two very iconic buildings were destroyed and people around the world lost their sense of security, because suddenly, a new threat arose, that could strike anytime, anywhere, at anybody (considering that WTC was in the middle of the most powerful country in the world). I think that in case of Pearl Harbor, people more or less expected it could happen, considering that Europe was already at war and Japan wasn't on the side that had US support. Not to mention 9/11 was aimed at civilians, not military. Soldiers (at least in theory) have "death in defense of their country" is a part of their contract and Pearl Harbor was a strictly military target, so Japanese attack was a logical thing to do (relatively speaking). 9/11 was caused by irrational beliefs, by people who simply wanted to kill as many people as possible (showing that they didn't even looked into Koran, which they value so much).