A lot of people did not take Hitler and the Nazis seriously in the early 1930's. I can't find the quote just now but I've read some political leaders at the time who said that they expected Hitler to moderate his policies once he had power consolidated.
ISIS seems different from other terrorist groups in that they are seizing and controling territory (or attempting to). Their stated goal is to reestablish the Caliphate as an actual government over the captured land. They may be gone someday, but we don't know what its going to take to do that.