As posted by others:
1. Arab governments using the US as a Scapegoat
2. Poverty
3. Fundamentalism
4. The US's historical backing of dictators
5. The US's toppling of democratically elected governments
6. Ongoing warfare
7. The US's continues meddling (regardless of the intentions) in the middle-east.
Did I miss anything?
I think that all of these factors has created an atmosphere of resentment. The Youtube video is simply being used as a rallying cry. It's simply the latest slight (either real or imagined) that many Arabs have had to put up with in their life times. And so a small number of Arabs (urged on by fundamentalist factions) are rioting.
If this was a pan-arab/pan-muslim movement as Aretas' orginal post suggested "millions if not hundreds of millions of people" then there would be more American deaths, more embassies destroyed and more people on the streets. Hell if "millions or hundreds of millions of people" wanted to do anything then it would simply be done.
This unfortunately is the disenfranchised lashing out with deadly effect.
Is the answer violence? No. But I've lived through riots and seen mob mentality first hand. There is no rhyme or reason, only hatred and the fear that the mob might turn on you.
Just my 2-cents.
-TM