I would argue that 9/11 was not terrorism, in the way that blowing up random Jews in a pizza parlour is terrorism. The targets were places directly involved in the machinations of the Great Satan, either military or financial.
Wrong, 9/11 was just as much terrorism as blowing up random jews. The targets were chosen to completely enrage, horrify, and terrorise USA into disproportionate or collective punishment thus further radicalising the terrorists' actual or potential supporters, who might otherwise be repulsed by the carnage that such extreme violence causes. This link has the complete article that argues logically and convincingly that Terrorists have embraced suicide attacks mainly for their advantages in this world, rather than their rewards in the next
like other forms of terrorism, suicide attacks are designed to impress the terrorist's own constituency, as well as to coerce their opponents—and “martyrs” have a particular propaganda value
By picking on democracies, the terrorists can be reasonably sure that their adversaries will stop short of “the Mongol method” (ie, wholesale slaughter of the population from which the bombers derive).
The terrorists can also look a few moves ahead and "misery all round" will work in favour of criminal groups like terrorists.