This short video explains why Americans, and a lot of other people, should be very concerned about Islamism. The video focuses on Islamist extremists and their desire to impose sharia on the entire world, but there are large numbers of Muslims in America who are not extremists who nevertheless would gladly exchange the Bill of Rights for sharia. According to one study 51% of American Muslims would prefer to live under sharia, and 25% of American Muslims think that violence against those who insult Islam is justified.
Clearly, we have a problem. After all, what constitutes an insult? Drawing caricatures of Mohammed? Refuting Islam's historical claims? Believing that the Koran is a plagiarized Old Testament? Actually, most Muslims would consider all of these to be insults and if 25% believe insults deserve a violent response then our basic freedoms of speech and the free exchange of ideas are in jeopardy.
Anyway, watch the video. It's not long and it puts the problem into perspective: