His motives are very simple really, he doesn't like us.  He hates our Rock and Roll, the fact that we don't veil our women, he hates Baywatch, Britany Spears, and what he percieves to be a decadent godless culture that props up what he considers false islamic regimes in Saudi Arabia, Egypt, Jordan, and others.  Like all extremists he thinks that his way is the only way and that all others not like him should either become like him or vanish.  

With the attack he hoped to wreck our economy strike fear into our hearts and ruin our way of life.  He has of course failed. He has failed because he does not understand us, and with his limited view of the world he can't understand us.

I've given this topic some thought, and I think the responses contained herein are spot on.