Islamic invasions and Islam piracy plunged Europe into the Dark Ages.
The culture and economy of Spain and Sicily was devastated and a giant migration of Christians and Jews into Northern Europe ensued.
The Muslims introduced the concept of the holy war (Jihad) and antisemitism into Europe.
Europe was cut of from the libraries and universities around the meditereanian and papyrus productions, which created a downfall in the intellectual and spiritual culture of europe until the 9th century.
Muslim and Mongol invasions hastened the discovery of the New World and forced Europeas to circumnavigate Africa to get spices from Asia. (only positive point, but this was after the Middle Ages)