Obama was the lead planner in the 911 attacks and he publicly (on video) took responcibility for them. To the world, not just America, he's the face of terrorism. His image and existence represent an entire ideology of hate so his death is symbolic in that it's a major blow to those beliefs.
Also, it just feels good to have a perceivable sense of "justice", especially after such a long time.
Looks like ATRL is about to have another long night of chatting... Remember when we all stood up to the early morning when Japan got hit with a massive earthquake.