I have a similar opinion to Hillary Step's: A key difference between Christianity and Islam is how they deal with political matters. Christianity in its beginnings left them to the secular authorities and early on its followers believed that it was up to God to bring about eschatological change. It had a bearing on how Christians acted and reacted in their surroundings. Mind you that notion swung back and forth throughout history, but like Hillary Step said its been adapting along the way, mostly in favour of a more tolerable and less dogmatic stance towards unbelievers.
Islam from its inception, puts the onus on its followers to bring about the theocracy of God on to the entire world. At first, there was a need for self defense, so they engaged in battle, but that quickly changed to warring for its spread. They would also have to make the often ugly choices that authorities and states have to make under difficult situations. I think if they alter the notion of the theocracy (spiritual vs. concrete reality), or at least the extent to which they can morally bring it to others, the religion could be a very positive force. Like Little Toe pointed out, they need time.
Editted to add: I may be confusing this with another, but from what I recall the marriage of Muhammed to that girl was a way of forging ties with her father's tribe. They wanted Muhammed to take her as his wife and he held off until she was ...again IIRC...menstruating. I personally find it disturbing, but at least thats a bit more of the story.