On the extreme range of the "not much of a worry" to "yikes", one end is:
Dumb Terrorist Theory:
Another theory is the Melting Pot:
Nearly eight years after the attacks, it remains physically sickening to review these for-want-of-a-nail details about what the U.S. government knew prior to 9/11. The various intelligence agencies' failures to pool their knowledge about the plot should surprise no one familiar with Washington's bureaucratic culture. But it's equally true that to count on so extreme a degree of government dysfunction, as al-Qaida effectively did, was foolhardy in the extreme. The terrorists took an unacceptably high risk that they'd get caught, and, just barely, they beat the odds. That they succeeded does not prove they were smart to try.Nor is it clear that they were smart to succeed.
Whatever the reason, American Muslims appear far less inclined to support the global jihad than their European counterparts. In the United Kingdom, 81 percent of Muslims consider themselves Muslims first, British second. In the United States, only 47 percent consider themselves Muslim first. After 9/11, 58 percent of American Muslims said they approved of President Bush's response to the attacks. In a 2007 Pew survey, 51 percent said they were very concerned about the rise of Islamist extremism, a proportion the report termed "much greater than the concern expressed by Muslims in most of Western Europe, the Middle East, and elsewhere."Burden of Success Theory
According to this theory, the 9/11 attacks were so stunning a success that they left al-Qaida's leadership struggling to conceive and carry out an even more fearsome and destructive plan against the United States. In his 2006 book The One Percent Doctrine, journalist Ron Suskind attributes to the U.S. intelligence community the suspicion that "Al Qaeda wouldn't want to act unless it could top the World Trade Center and the Pentagon with something even more devastating, creating an upward arc of rising and terrible expectation as to what, then, would follow."I always got the sense that AQ did not do minor stuff in the US because it is entirely about mass carnage. Even if they cannot exceed 9/11, the challenge of doing something very big might be prohibitive, whereas something much smaller would be possible but not suitable.
More interesting stuff in the piece. Read it.