Over four years after we began the war on terror, al Qaeda demonstrated that they still can manufacture events (black swans) that have the potential to throw our dynamically unstable global system into chaos. We appear be more vulnerable today to shocks of this type than we were four years ago.
The first attack, a successful one, on the Askariya shrine in Samarra, caused so much friction in Iraq's social systems, the country will likely end up in civil war. This would put the entire US venture in the country at immediate risk as the situation deteriorates. The second attack was an abortive attempt to take down the massive Abqaiq facility in Saudi Arabia. This was almost a disaster for the global economy (if the Aramco uniformed attack team had breached the second security ring, we would be looking at $100 a barrel oil today).
More is on the way. We should expect al Qaeda to begin to disrupt Saudi oil production in the more efficient but less glamorous method we have seen in Iraq and other locations, now that the attempted coup de main on Abqaiq was thwarted. These smaller and less difficult attacks, can in time replicate the effects of taking down a portion of Abqaiq. There will also be more attacks on Shiite symbolic targets, since even with the diminishing returns on terrorist attacks post-Askariya, it won't take much to continue the fragmentation.
Remember, al Qaeda is open source now. It is operating at a level of decentralization that will allow it to take the war to many locations simultaneously without central management. Bin Laden's threats reflect his inside knowledge of what is now a self-directed movement and not planning he has directed personally. Despite all of this current activity in Iraq and Saudi Arabia, we should also expect to see systems disrupted against US mainland targets in the not too distant future. The organization has more than enough capacity to accomplish this.