From: Schneier on Security
Although the plot was disrupted before a particular airline was targeted and tickets were purchased, al Qaeda’s continued attempts to attack the U.S. speak to the organization’s persistence and willingness to refine specific approaches to killing. Unlike Abdulmutallab’s bomb, the new device contained lead azide, an explosive often used as a detonator. If the new underwear bomb had been used, the bomber would have ignited the lead azide, which would have triggered a more powerful explosive, possibly military-grade explosive pentaerythritol tetranitrate (PETN).
Lead azide and PETN were key components in a 2010 plan to detonate two bombs sent from Yemen and bound for Chicago—one in a cargo aircraft and the other in the cargo hold of a passenger aircraft. In that plot, al-Qaeda hid bombs in printer cartridges, allowing them to slip past cargo handlers and airport screeners. Both bombs contained far more explosive material than the 80 grams of PETN that Abdulmutallab smuggled onto his Northwest Airlines flight.
With the latest device, al Asiri appears to have been able to improve on the underwear bomb supplied to Abdulmutallab, says Joan Neuhaus Schaan, a fellow in homeland security and terrorism for Rice University’s James A. Baker III Institute for Public Policy.
The interview is also interesting, and I am especially pleased to see this last answer:
What has been the most effective means of disrupting terrorism attacks?
As with bombs that were being sent from Yemen to Chicago as cargo, this latest plot was discovered using human intelligence rather than screening procedures and technologies. These plans were disrupted because of proactive mechanisms put in place to stop terrorism rather than defensive approaches such as screening.