I would say, the crusades have played a great role in creating hatred between world's religions especially Christianity and Islam. It is normally thought of as a basis for every difference that exists between these religions today.

 Although I agree with Aicha, about the Crusades causing the hostility between Islam and Christianity, I think that the crusades taught us that there is much to be gained when civilizations and cultures merge. I would credit The Crusades with bringing Europe out of the dark ages. The Arabic and far Eastern cultures accessed by the 'silk' road, were far more advanced than Western Europe. Trade with these entities brought much to the European Culture and Sciences. The Crusades had more to do with money then they did anything else including Religion.