I think one of the most devastating things that terrorists can do is a focused strike that would scare the shit out of city people and almost bring our economy down by our over-reaction to it.
Like what happened after 911 due to the actions of our favorite cretin.
I think actually killing people is only ancillary, they want maximum effect on our whole society. A few envelopes of anthrax had us doing backflips and we still consent to getting fondled every time we travel so as to remind us that there is someone that we should be pissed at.