Peter Beinart has a terrific column on how and why al Qaeda has gotten so much less dangerous:

Once upon a time, al-Qaeda's modus operandi was to launch multiple, simultaneous attacks. That way, even if one attack failed, the entire operation wouldn't. On 9/11, the network deployed 19 hijackers on four planes; on 12/25, by contrast, it managed only one. Second, the underwear attack failed because Abdulmutallab wasn't particularly well trained. The 19 Sept. 11 hijackers were personally selected by Osama bin Laden from the tens of thousands of potential killers who went through al-Qaeda's Afghan training camps in the 1990s. The ringleaders got extensive training on the design of airplanes and the behavior of aircraft crews, even before they enrolled in U.S. flight schools. The grunts were made to slit the throats of camels and sheep to overcome their inhibitions about murder. Abdulmutallab, by contrast, reportedly used a syringe to try to detonate a notoriously hard-to-detonate explosive called PETN. "To make this stuff work," says Van Romero, an explosives expert at New Mexico Tech, "you have to know what you're doing." Abdulmutallab, it appears, did not....

All this means that even in places like Pakistan and Yemen where al-Qaeda or its affiliates retain some organizational presence, it is much harder to train lots of would-be terrorists for complex, mass-casualty attacks. In response, al-Qaeda seems to be relying more on solo operators, people like Abdulmutallab, Fort Hood gunman Major Nidal Malik Hasan and Najibullah Zazi, the Afghan American arrested last year for allegedly plotting to blow up buildings in New York. These lone wolves are harder to catch, but they're also less likely to do massive damage.