Remember Archduke Ferdinand at the start of WWI? It's fears of something like that, I think.
I'm far from the best candidate to give an accurate analysis of the situation, but from what I understand, Bhutto was everyone's last great hope to stabilize and modernize that country. Her assassination could galvanize Pakistan's populace into action against an oppressive Islamist minority seeking to gain control of the country. But, most likely, the people will just submit. It's what they do.
So we're left with a sort of bad guy in charge of the country, who's pretty shady on his best day, and a Taleban-friendly militant Islamist tribe (read: Bullies) in the north looking to spread its influence throughout the region by way of violence and terror, and the one leader who the people felt was worth a damn gets killed by some **** working for one of those minority factions. It's very unsettling to find out that someone so strong and well-liked and capable of leadership could be or would be eliminated so easily and indiscriminately. Even Bush, typically so cocky or smug on-camera despite his bumbling history with public speech, appeared shocked and nervous while giving his comments on the matter.
So, yeah. The world's holding it's breath. That's why the stocks have dropped. (I, personally, am thinking of fleeing to Canada.)