Firstly if Osama has been running around in hiding for the last say 10 years, then it's unlikely that he has been an actual tactical or strategic leader for the movement. He may be a leader in name but most likely not in practice. Therefore it most probably won't leave such a big vacuum or lead to a great power struggle. The same people who have been leading the terrorist forces in his time of hiding will most probably continue to do so with little problem.
Secondly the terrorist movement needs no great motivator, nor extremist leader. The greatest cause of extremism against America is American foreign policy which includes the proping up of dictators, the overthrowing and usurping of legitimate governments, parisan support for Israel and it's unhealthy interest in oil mainly from Arab countries. So there will be no lack of extremism in the absence of this man.
Thirdly if anything the U.S. has most probably made this man into some hero martyr figure now that he and his son has been killed. The fact that he and his son has been killed may make him even more attractive to Shia muslims since a lot of their theological and historical baggage is tied up with Husain who was a muslim martyr seen to be fighting an oppressive movement.
Finally in a broader look at this, lets look at the trade off that has been made by the US. Had America not invaded Afghanistan (and carried out extensive military operations in pakistan) to kill this guy and shut down al-qaeda then this security situation and the problem of fanatic Islam would have been a much more isolated and smaller problem. The irony is that in seeking to protect itself from terrorism by invading and occupying afghanistan and attacking locations in pakistan over the last decade, these two acts (among others) have instead lead to a sharp and steady increase in islamic terrorism against the US and they have been the biggest motivation for young men to become islamic extremists.
So in conclusion, although this may seem like a huge victory for the US, though this may increase troop moral for a short time, really this is a minor factor in the huge festering problem that the U.S. has created itself. It's approach in trying to find and kill this man has been hugely counter-productive.