Michael Bloomberg said of the aborted terrorist strike in New York City, "We are very lucky." Lucky, sure. But very lucky? The last two attempted mass-casualty terror attacks failed largely because the terrorists were incompetent. The New York City terrorist, among other things, seems to have used non-explosive fertilizer for his bomb.
Obviously terrorism remains a danger, and we can be fairly sure that various levels of counter-terrorism have spared us from numerous attacks. That said, I think Americans have a general tendency to overstate the capabilities of terrorists. Movies and television have accustomed us to the notion of diabolically brilliant villains:
In reality, it's very rare to find people who are both sociopathic mass murders and capable of carrying out a complex, technically challenging plot. Most terrorists are basically dolts. They can do a lot of harm when they enjoy physical proximity to their targets and chaotic or loose security environments.
Again, we shouldn't rely on their incompetence to protect us, but neither should we assume that terrorist incompetence is some unusual event. Terrorists would like to enjoy the romantic image of brilliant tactical warriors. The brilliant people involved in this fight are almost entirely on the side of democratic governments, and they're working to thwart the terrorists. That's where you find the complex strategic thinking and technical sophistication. The terrorists themselves are merely simple thugs.