Zawahiri was basically asking for it. He had been appearing in many videos and releasing many audio statements in the past year. Each time he did this he took a risk, because there is always the possibility of tracing the origin of the statement, no matter how careful they were.
He was also asking for it by falling into a pattern. Early reports say that Zawahiri had frequented safe houses in the Bajaur region last winter, and he returned this winter. A regular pattern is death to a terrorist. If he came to the same place again at the same time, he certainly should have known better.
Naysayers will rush to point out that this will not spell the end of Al Qaeda, and will point to people like Zarqawi in Iraq as the real threat. But the fact remains that Zawahiri was the spiritual leader of the movement, and important both symbolically and operationally. Indeed he may have been the primary leader for the last year or so. Others may try to take his place, but there will never be another like him; he was Osama’s right hand man. So getting him shows that the drive to bring the 9/11 criminals to justice continues.
Finally, if Osama bin Laden is still alive, he has lost his most important mouthpiece, the only person who had the legitimacy to speak in his name. Anyone else who does so in the future will have dubious authority — who knows if they really are in contact with Osama? This may goad OBL to surface (and perhaps suffer the same fate), or exploitation of the intelligence found at the site we reveal he is dead. Either way, good news.
If we got him that is.