Islam is not a threat any more than Judaism or Christianity is a threat. But those who believe that force should be used to make others follow their religious views are a threat. Those who believe that governments should enforce purely religious views are a threat.
In general, anyone willing to use force based on his religious views is a threat, whether those religious views are Islamic, Christian, or Wiccan.
It just so happens that there are more such people with Islamic views. I don't think this is because of any special feature of Islam. One can easily imagine (and need only look to the crusades to see) similar Chrisitian threats.