This is a good point. On one hand, they were astute enough to find and take advantage of some of our weaknesses. On the other hand I think it was an easy operation for them to accomplish.
The tangos of 9/11 conducted this operation without hardship. They walked through our ports of entry and lived in comfort. They planned and prepared free from scrutiny. Despite the hightened security it is still relatively easy to bypass border security. Yet the tangos can't handle it. IMO, if they had any tactical or strategic competence there would have been more attacks all over America.
My take is they want to hit us *hard*. I don't think they are interested just numerous multiple little hits.
Using 911 as their MO, I'd say they have a big appetite for cataclysmic, brutal, economic killing, front page surprise operation's, where *many* thousands are killed.
Remember watching 911? It was like watching a Superman movie where the bad guys were actually knocking down huge skyscrapers. I don't think these guys think in quantity, as much as quality.....They know they have to make their hits count.