There is a not-insignificant difference between religious imperialism and suicidal extremism. Muhammad and his political heirs were conquerors, sure, but they were also nation-builders and city founders who saw themselves as cultural successors to the Egyptians, Greeks, Persians, Romans, Christians, Jews, etc.  On average, they were also more tolerant of other religions than their European contemporaries and modern Islamic theocracies. The biggest difference in my eyes is that modern extremists don't seem to be interested in conversion at all, only destruction - the fact that Muslims themselves are by far the biggest target of extremist violence is evidence enough of this.