If I were a terrorist leader and I wanted to strike the United States, I would not focus on huge attacks the way Al Qaeda seems to be. I would try to carry out a large number of small attacks that would throw the country into chaos.
It is true that spectacular attacks like the 9/11 attacks get a lot of attention. It is true that they hurt the US. However, I would argue that continual small attacks would be much more damaging to the country.
If I were a terrorist leader, I would have my people fan out across the country, committing small attacks in a variety of places. I would have them leave bombs in fast food restaurants near to freeways. I would have them leave bombs at gas stations. I would have them leave bombs on city busses. I would not go for “high value” targets. Instead, I would just have them go for a large quantity of attacks.
By doing this, I would hope to paralyze America with fear and I would hope to do so in a way that is much easier to arrange than a 9/11-style attack. As it is, people in most of America can feel that Al Qaeda is never going to hit them because they do not live near a major target. If, by contrast, they were made to feel that a bomb could strike anywhere at any moment, it would create a great deal more fear and would hurt the economy more than large strikes can.